access_token: is your secret key used to access the user's data. It can be thought of the user's password and username combined into one, and is used to access the user's account. It expires after 1 month.
refresh_token: is used to request new access_tokens. Since access_tokens expire after 1 month, we need a way to request new ones without going through the entire authorization step again. It does not expire.
According to Imgur API docs:
https://api.imgur.com/oauth2#refresh_tokens