Steemit is not a normal social network. It's powered by Steem—a blockchain with no central owner, run by community members around the world. Steem is no ordinary blockchain. Unlike Bitcoin or Ethereum, anyone with an account can transact on it for free.
To make a decentralized blockchain fee-free for everyone, Steem uses a system of Resource Credits (RC) to ensure fair usage.
Each person gets free RCs based on their STEEM Power holdings. The more STEEM Power you hold, the more RCs you're allocated. You consume RCs each time you do an action that writes to the blockchain. Your depleted RC reserves automatically regenerate back to full capacity over a 5-day period.
This system helps to cover blockchain processing and storage costs. It prioritizes posts from people with a longer term interest in the platform. It also protects the network from spam.
To keep interacting on Steemit, simply wait until your RCs regenerate. Alternatively, get more RCs instantly, by powering up STEEM.
Note: Community witnesses upgraded the Steem blockchain earlier today. Some people are experiencing trouble posting and transacting while the new RC system stabilizes. This is due to the negative RC issue outlined by 
