The action describes how easy or hard the keys are to actually press down to produce a note. Real pianos have weighted keys and, therefore, quite a heavy action, so for beginners, a more lightly-sprung ‘synth action’ keyboard is more likely to fit the bill to start with.
A light action is often better for small hands in particular, but don’t be too tempted by mini-size keys – little hands can quite easily handle full-size keys as long as the action isn’t too heavy, and learning on full-size keys will remove the need for switching from mini to full-size at a later point, which can be difficult once you’ve got used to small keys.