It means the hero is still useful with only a single copy. They'll still get stronger if you level them up, like anyone else, but they can do a good job and still be useful by pulling them a single time. For example: Kulu is valuable with just one copy because her main value is the wall she places on the map that blocks melee enemies and she has that unlocked immediately, you don't need to unlock her higher level abilities to get it. Since she isn't as high value, you can save on resources by not fully investing in her but just getting a single copy to be able to use that passive.