what does "Reject (revise and resubmit)" mean?
Revise and resubmit means exactly what it says: if you revise the manuscript and resubmit it, we will look at it again (hopefully with the same editor and reviewers, but not necessarily). Typically this means the required revisions are substantial enough that it will go to reviewers again.

Reject means we do not want to see the manuscript again.

More recently reject and resubmit has become a thing. It is like a revise and resubmit, but the journal is going to count the manuscript as a new submission so their rejection rates go up and the time to final decision goes down. Sometimes they try and use the same editor and reviewers and ask for a rebuttal letter, just like a revise and resubmit. Other times it is on you and they treat it like a new submission.

This is clearly a revise and resubmit. If you wish to not revise and resubmit you can just submit it to another journal without telling them anything, but it would be polite to tell them that you are not going to resubmit the manuscript. That way they can close out the paperwork on their end. As for concerns about double submission, once a decision, apart from acceptance, is made, you can do whatever you want.