This is a tiny PCB with a single-gate 74HC125 buffer on it! It's a tri-state buffer with an enable line. If that doesn't tell you much, here's an explanation.

The 74HC125 IC has an "enable" (EN) line, input and output. When EN is set low, signal on output is the same as signal on input, no matter if it's high or low - in other words, output will follow the input, when input has 1, output will also have 1, and when input has 0, output will also have 0. When EN is set high, however, output is set to high-Z - meaning, it's not being driven either low or high, no matter what's on the input.

Effectively, this buffer allows you to "interrupt" some kind of digital signal when you want that signal to not influence something. Typically, I use it for SPI and UART, but there's a lot of ways you can use it beyond simply, say, tri-stating MISO signals.

This breakout is breadboard-friendly, self-documenting (using silkscreen), open-source and immensely useful. Usecase examples:

• Working with SPI devices that don't have a chip select pin. With this breakout, you can just use the CS line to switch the CLK signal to your CS-less SPI device - as a result, you'll be able to share your SPI bus with other devices and have it all work flawlessly.
• For example, certain SPI OLED (SH1106 and SSD1306) breakouts do not expose the display's CS pin - so, if you want to put some other device on the same SPI bus, the OLED will receive all the data directed to that device, and display a lot of garbage.
• Working with SPI devices that don't tri-state their MISO outputs. This is a known problem that can break your SPI bus as soon as there's a single SPI device misbehaving, you will receive garbage every time you try to read data from a device. In this case, you can use the CS signal of the module to interrupt the MISO line coming from the module, so that, while the device is not being talked to, it cannot ruin the bus for every other device.
• For instance, it seems like popular RC522 RFID reader modules have this problem - as soon as you try to use multiple RC522 modules in parallel, you will receive garbage data unless you tweak your hardware.
• Putting multiple devices on the same UART port - in case you want to connect more than two transmitters to the same UART RX line, this module allows you to effectively disconnect one of the transmitters while you don't want the data coming from it to interfere with data coming from the other transmitter.
• For example, say you want to connect a Bluetooth dongle and a programmer to your Arduino at the same time. When you have a UART-connected Bluetooth on your Arduino, it will try to drive its TX line (connected to the RX line of your Arduino) all the time - and so will the Arduino's onboard USB-UART chip. As a result of this competition, you will receive garbage on your serial port, and you won't be able to re-flash your Arduino until you physically unplug it from your Arduino. A much better alternative is buffering the connection betweeen the BT module's TX pin and Arduino's RX pin, then connecting the buffer's EN line to a GPIO on your Arduino, and then only enabling the buffer when you actually want to receive something. That way, you'll be able to program your Arduino without having to disconnect the BT module all the time - since the BT module's TX line will effectively be disconnected in hardware.
• You can also, say, connect multiple GPS receivers to your Arduino using only one serial port, to i.e. improve precision (assuming that even makes sense) or study differences between different GPS receivers.
• 5V=>3.3V level-shifting - this buffer can accept 5V input signals while powered from 3.3V and outputting 3.3V.
• So, if you to want to send data from a 5V Arduino to ESP32 over UART, this board will protect the ESP32 from getting 5V on one of its pins. When you use this board for level-shifting, you don't even need to connect the EN line to any GPIOs if you don't need the "interrupt" function - just connect it to GND (a solder blob between GND and EN on the breakout will work, too).
• Reverse-engineering electronics devices and making them work the way you want - interrupting an output of some IC to inject your own signals, or just to inhibit something from happening
• For example, imagine you have a "smart" PIR sensor that sends you push notifications when it detects movement, and you want to inhibit it from triggering sometimes. In that case, you only need to find a PIR output line inside the sensor, then interrupt it with this buffer and control the EN line from some other method - for example, a MCU, or an optoisolator's output.

IC used: 74AHC1G125. There's a 100nF capacitor in parallel with VDD, and a 10K pullup on the EN line.