A relationship that is "on reserve" is one in which there is mutual love or attraction, but one or both parties is not ready for a relationship for one reason or another. They will then be "on reserve" until both members are ready for a relationship.
Relationships on reserve often, but not always, are of the friends with benefits or friends with minor benefits category. In this case, such a relationship may be called "friends with (minor) benefits on reserve".
Bob: Is Sally your girlfriend?
Jack: Not really. She just got out of a bad relationship and isn't ready for another one yet.
Bob: Do you guys have feelings for each other, though?
Jack: Yeah; we're on reserve for now.