I don't "mean" anything. It's a technical term. Another name is "free wheel diode".
All inductive loads create bad sparcs, or over voltage, when they are turned off. Those diodes suppresses that. Those sparcs easily tricks electronics to go wrong, like a reset, or even get damaged.
5 volt pump powered by that 4.5 volt battery is ok. But, do some Google on those diodes and connect one.