After I back up my files to the air port time capsule can I erase my files from my MacBook Pro ?

No. Time Machine is not an alternative storage system, but a sophisticated archival backup system. Deleted files on the main drive will eventually be removed from the backup. If you need storage, then use a separate drive.

To add a bit to what Kappy has said, it is important to understand that Time Machine backs up the changes on your Mac.

If you delete a file from your Mac, Time Machine pick that up and delete the same file from the Time Capsule....and you will not know when this will occur.

Even if the file might stay on the Time Capsule for some time.....since you deleted the file from your Mac, the only copy of the file that you have is on the Time Capsule.

Ask yourself what will happen if you have only one copy of your valuable data on the Time Capsule....and the Time Capsule has a problem.