A barometric sensor is a tool for measuring the pressure of the air. Pressure is the amount of force per unit area, and even though we can't feel it, air pressure is always acting on us.

Barometric sensors are used in drones to figure out altitude because air pressure drops as elevation goes up.
When a quadcopter drone can figure out how high it is, it can stay in the air at the right height. 

Because of how barometric sensors figure out altitude, they need to be carefully built into a drone so that they aren't affected by sudden changes in air pressure.

For example, if the wind hits the barometric sensor directly, it will show a higher air pressure than is correct. If the air pressure is measured wrong, the quadcopter drone will rise or fall to the wrong height.

So, the barometric sensor needs to be protected from the outside environment, and air needs to flow over it in the right way to avoid getting wrong pressure readings.

Most of the time, gusts of wind make it hard for a barometric sensor to measure altitude, but humidity and air temperature can also slightly change its readings.