Constant Bearing Decreasing Range
When another ship or boat is CBDR, their bearing doesn't change and they keep getting closer and closer. It's not rocket science
-- if this happens, you're on a collision course
. For a ship dirver, this is bad news, and you need to either turn, speed up, or slow down.
This is an example of relative motion-- when they're not CBDR, they'll have what's called bearing drift, where the other vessel moves either to the left or right. Navy folk use maneuvering boards to plot other ships' relative motion and find out their course, speed, and CPA (Closest Point of Approach.)