AGND VS DGND on PCB?
Source: Where to put regulator when separating analog and digital grounds?

Some designers like to separate digital and analog sections. There are some parts that even have an analog ground and digital ground. This is to prevent return currents from devices in the digital section creating 'ground bounce' on sensitive devices in the analog section. 

In a schematic, usually the grounds will be indicated with different symbols and net names on the schematic, and a net tie is used to join them together.

KiCAD 5 --- what is the significance of the various GND symbols?