Well, you give them the right to do whatever they want as is common with similar services and social media, but you retain the ownership of the content. I don’t know of any service’s that take away your ownership and I am not even sure that’s possible in an agreement like this. Don’t quote me on the last part though.

From their service agreement:

You retain any ownership rights you have in Your Content, but you grant Reddit the following license to use that Content:

When Your Content is created with or submitted to the Services, you grant us a worldwide, royalty-free, perpetual, irrevocable, non-exclusive, transferable, and sublicensable license to use, copy, modify, adapt, prepare derivative works of, distribute, store, perform, and display Your Content and any name, username, voice, or likeness provided in connection with Your Content in all media formats and channels now known or later developed anywhere in the world. This license includes the right for us to make Your Content available for syndication, broadcast, distribution, or publication by other companies, organizations, or individuals who partner with Reddit. You also agree that we may remove metadata associated with Your Content, and you irrevocably waive any claims and assertions of moral rights or attribution with respect to Your Content.

I don’t really understand the part I made bold, so if anyone could explain it (optimally with credible sources) that would be great :)