I had an expecially tough time fighting the Romulan ships with this designation. I'm a Lt. Commander 5, and it was extremely difficult, although the ground mission wasn't that difficult at all. I got the impression that I need to level to Commander before I attempt another mission against Romulans. I have all level IV equipment on my cruiser.
After accepting a mission, press L to bring up for ship log, and click on Episodes. it'll tell you what rank the mission is intended for. Remember, you can always drop the mission and do it at a later level if you need to!
Red "+" numbers indicate how many levels above your current level that enemy is. If you are well equipped and know how to handle yourself you can beat them, but it wont be easy.
White "-" numbers indicate the enemy is lower level than you.
The Green "0" means they are the same level as you.
I fought off a Romulan cruiser with a red +3 designation, and a Orion cruiser as well. The fight with the Romulan cruiser took forever, and he almost go me before I disabled him. Now I know not to take any more missions from that Admiral at the alpha centauri space station until I level up.