The name is kind of self-explanatory - it prevents the default action associated with the event. 🙂
Taking your example here, we have an onClick event being associated with an anchor tag.
The default behaviour of a click event on an anchor tag is to take the user to the path specified in the href attribute. But that's not what you want to do here. You have an anchor, but you want to override the default behaviour and, instead of taking the user to / , you want to execute doMeow().
That's where event.preventDefault() comes in. It prevents the default event side-effect from occurring.
PS - On a side-note, this is not react specific. This is general javascript browser event method!