What is meant by "no memory effect" on NiMH batteries?

Answer 1

older batteries had a 'memory of charging' meaning:
If you let the battery run half way down and recharged it as a regular routine the batter would 'remember' that and only charge half way no matter how long left on charge. Those batteries required being drained all the way before charging to maintain full charge capability.
Newer batteries today have no memory and you can charge them at any level with no ill effects.

Answer 2

The battery does not need to run totally down before recharging. Older technology would cause the batteries to not charge as deep as the original charge if not completely discharged.