what does it mean to ground something?
A battery has a + and a - pole.
Ground is the reference voltage, usually defined as 0V, or - as on the battery.
If you have two circuits (say one for a sensor, and another for the microcontroller), they usually need a reference voltage so that they can work together. This would be referred to as grounding. Forgetting to ground circuits is a very common (and the most likely) mistake among beginners, so its why you often read 'don't forget to ground your circuit!'
It is called 'ground' because back in the old days, dirt (earth) was considered the reference voltage.