Transactions on the EOS blockchain are free, but they require staking an amount of EOS and CPU to the network.
The more transactions you perform on EOS, the less CPU you will have available. This means that you can run out of CPU and will not be able to make new transactions until you stake more CPU.
CPU can take up to 24 hours to regenerate depending on EOS network congestion.
When converting EOS tokens on the Bancor network you will receive an "Insufficient CPU" notification if you are trying to process a transaction and you do not have enough CPU:
As shown above, there are three ways to get more CPU:
1) Wait for CPU to regenerate (may take up to 24 hours)
2) Get Emergency CPU. (Various third-party services will lend you some CPU.)
3) Stake more EOS to generate additional CPU.