Can someone please help me understand COG in FRC

COG stands for Center Of Gravity. It is basically the point where the mass of your robot is distributed on average. More weight at the top of your robot means a higher COG, which means your robot has to tip to a smaller angle before it will completely fall over.

old paper I wrote. Might get you some of the basics.

In some cases, people use COG to mean Center of Geometry instead of Center of Gravity. You will need to consider the context to determine what the writer is referring to. Here is an example where COG is referring to Center of Geometry.

In addition to the height and the possibility of tipping, the lateral location of the CoG (gravity) is also important when figuring out what a robot will be like when turning or being pushed into a turn. If the CoG is essentially over one axle of a drop-center drive, the other wheels will carry little weight and the robot will turn very easily, but also be easy for other robots to cause you to spin by pushing you away from the CoG axle. If the CoG is squarely between two axles, you may have a bit more trouble turning (though if the length of that segment is significantly less than the track width, you shouldn’t have a problem) but you will also be more resistant to being turned externally.