Leaving a copy of the retrieved message on the server
If you'd like to keep a copy of each message Gmail retrieves in your other mail accounts, select the Leave a copy of retrieved messages on the server checkbox. This way, you can access mail in your other accounts, and in Gmail.
Please note that some email services may still delete your messages when Gmail fetches them. If this option is checked, Gmail will instruct the other domain to leave the messages in place, though the final behavior is up to the other domain.
If you'd rather delete copies of messages that Gmail retrieves, leave this option unchecked. Gmail will delete the messages from the other account as those messages are retrieved, meaning you'll only be able to read them in Gmail.