What is Odometry and how can my team get started?

HI everyone. My team just qualified for worlds and were wondering how we could improve out autonomous as it was very unreliable. We are currently are using PID but we feel like we need to still fix some things. We already know we want to implement tracking wheels to reduce wheel slippage, but we still want some more accuracy and learned that odometry may be the solution. Any help would be greatly appreciated!

Here is a somewhat similar thread.