There are 3 types of resources that need to be considered when working on EOS: RAM, CPU, and NET.
In order to process a transaction, you need to make sure you have sufficient amount of staked CPU and NET, and that you own enough RAM to cover for the transaction, if you do not have enough resources, you will receive one of the error messages as seen in the screenshot below:
Click here to learn more about CPU, RAM, and NET.
Click here to learn how to add CPU, RAM, and NET to your account.