This multi-purpose sensor is used to measure temperature, relative humidity and barometric pressure. It operates as an I2C slave device.

The actual sensor is tiny but a board is available from China with the I2C connections and power on a standard 0.1 inch header which makes it easily accessible.

The device uses I2C to communicate with a host master and with the Pi, the topmost pins on the GPIO header are utilised to perform this.

    +---P1---+
3V3 01       .
SDA 03       .
SCL 05       .
    .        .
GND 09       .
    .        .
    /        /
    .        .
    +--------+
    
Relevant I2C pins shown, others omitted.

With the miniature board, only four wires are required to hook up the BME280 to the Pi.