What does a combined environmental sensor such as the Adafruit BME280 measure, and how does a microcontroller read it?

Question

Accepted Answer

The BME280 measures temperature, humidity and barometric pressure, and the microcontroller reads the values digitally over I2C or SPI.
Environmental sensing is the most common IoT workload, and the parts come in two styles:

Single-purpose sensors such as the RHT03 (temperature and humidity) or the TSL2561 / TSL2591 (luminosity, i.e. light level).
Combined sensors such as the BME280, which puts temperature, humidity and pressure in one package. Pressure also gives a rough altitude, since air pressure falls as you go up.

These are digital sensors: the chip does its own analog measurement and calibration and hands back numbers over a bus. The BME280 speaks both I2C (two wires, several sensors can share them) and SPI (four wires, faster). That is much easier to use than an analog part, where the microcontroller has to measure a voltage with its ADC and convert it to a physical unit itself.
A breakout board (the small red or blue PCB the sensor sits on) adds the voltage regulator, pull-up resistors and pin header so the sensor can go straight into a breadboard.
Go deeper:

Adafruit — BME280 breakout guide — pinouts, I2C vs SPI wiring and example code for exactly this sensor.