CPU and Bandwidth on the EOS blockchain is a bit complicated, but I'll do my best to explain it. Basically each transaction needs to use resources to process, and these resources are CPU measured in microseconds, RAM measured in bytes, and bandwidth that is also measured in bytes. 

By staking your EOS into either CPU or Bandwidth, you get your share of these resources, which can then be used to make transactions. Each transaction will cost you a little bit of CPU and Bandwidth, but they both regenerate over a period, so most users that just sends a few transactions most likely won't need to worry too much about them at all. 

So to summarize; CPU is a resource that is used to send transactions, and you pay a little bit each time.