To get space and use the chain, Hive uses "Resource Credits" (RCs). RCs are like gas or mana on other chains. A newcomer is gifted a small bit to get started and people are happy to delegate plenty of RCs to anyone generally interested in starting.

Then, as you earn rewards by posting/interacting/etc, your own RCs begin to recharge on their own. It's like a solar charging battery, you use the charge when you do stuff, but then it recharges gradually (if that makes sense!).