Staking and network resources
EOS is a "proof of stake" blockchain, which is different than blockchains like Bitcoin or Ethereum, which are "proof of work" blockchains.
To use EOS, you have to purchase EOS tokens, and set them aside, or "stake" them. Each token itself represents a small portion of the available resources on the EOS Mainnet network. The more tokens you have, the more access to the network you have purchased.
Access is broken down into three types: CPU (gives you access to network cycles to perform functions), RAM (gives you access to network storage), and NET (gives you access to network bandwidth).
When you stake you generally will want to stake to CPU and RAM. Most decentralized apps need those to perform the actions you request.
If you try to perform more actions than the tokens you have staked, you can "over charge" a bit. Once that is used up, you no longer have access to the network until the cool down period of 24 hours has passed. Within Scatter itself you can see your status by clicking on the key name and viewing the account itself.
If you find that you are often running out of one of the resources, you can purchase more EOS and stake it by hitting the "manage" button next to that resource.