ANDS is a mnemonic for remembering a particular behavior of a magnetic compass in the northern hemisphere which might be confusing. If you are flying along a straight east-west line at a constant speed, the compass will settle into indicating the correct magnetic heading. If, however, you accelerate, the compass will momentarily deviate to the north. If you decelerate, it will deviate to the south. This can be important if you're trying to navigate via the instrument, so you need to remember this:
Accelerate North, Decelerate South. A-N-D-S - ANDS.
Why does it do this? Well, something called magnetic dip and measures taken to mitigate its effects.