Power creep refers to new characters getting stronger over time, leaving older characters somewhat in the dust. A power creep can be due to a variety of things, but the most common reason is likely innovation. With new, better character creation, you get new, better characters. This tends to leave older characters out in the cold. This may lead to buffs for older characters.
The power creep in top lane is a bit of the same, though on a micro level. This refers to power within a single game. Top lane, more than others, tends to snowball because there is more 1v1 fighting and less help from the team (as there are no large objectives near top lane apart from towers). If one person gains even a small advantage in top lane, the advantage tends to grow and grow because one player is stronger and therefore can deny, kill, or out CS her opponent.