Well Beatgrid simply put is:
If the beatgrid is correct then the software will know when the beats are happening in a song. (Aka when the kickdrum beats and which beatcount it is).
Many features of the software depend on a beatgrid, for many most important the infamous SYNC button relies on it. But also some effects, slicer mode, BPM detection etc. etc. etc.
If you are using Serato you are in luck as with 90% of the tracks the beatgrid should be OK anyways and thanks to Serato’s “elastic beatgrid” it even holds true for songs with tempo changes. The others you can easily adjust, refer to your manual how…