It's normal for many young birds to leave their nests before they can fly. When they can stand up and perch (grasp with their feet), they hop out of the nest and spend time in the nearby trees and bushes and sometimes on the ground. Although fully feathered now, they don't yet fly well but should be able to flutter and climb to the lower limbs of trees and shrubs for safety. Fledglings are fed and tended by their parents until they become self-sufficient. Although the fledgling period can be a perilous time, you should understand that it's a normal part of the wild bird's life.
So, if you come upon a fledgling, don't be in too much of a hurry to rescue it. Use common sense. If it's uninjured, but in a dangerous place, move it to a safer place, close by. Remember, its parents will not abandon it because you touched it. If the bird is injured, or if after watching for a while you're sure no parent birds are in attendance, that's different; by all means rescue it.
It is especially important that you take any rescued nestling or fledgling to a licensed rehabber as soon as possible. Food and shelter, which you can give it, may keep it alive, but to have any chance of growing up to be "normal" and surviving in the wild, its greatest need is to grow up with others of its own kind. Only an avian rehab center can provide the right circumstances to properly raise an orphaned wild bird. So make haste, before you get attached to the bird, or worse yet, the bird gets attached to you.