Are you in an alliance? ask the leader as he/she ought to know how it works because they manage the reservation system for your team. if you don't have an alliance it is irrelevant and don't worry about it
oh dear, maybe you aren't in a good alliance? I guess i'll try and explain sense your leader has failed to do so. So when you reserve a city it puts up a sign on it that says "snawdog has reserved this city" that way your alliance members will know that you are gonna conquer (or are conquering that city soon) so they don't attack it and experience friendly fire...sorry i'm not the greatest at explaining but its better than nothing
If you go to the alliance page and click the reservations tab you should see an x beside your claim. You then click the x and select delete from the dropdown menu and the reservation will be deleted.