A Super Namek is a Namek who has ascended beyond the power of ordinary Nameks.
In the movie Dragon Ball Z: Lord Slug, King Kai states that Super Nameks have power and abilities 100x greater than that of any ordinary Namekian.
Picollo can be considered at the peak of a super Namekians power (around perfect cell level) he's reached a plateau and while still becoming stronger, he stopped gaining gigantic leaps. It is believed he can beat perfect cell with great effort. But is not even close to super perfect cell.
The hight of Namekian power was reached in universe 7, when all Namekians fused with nail, having the strongest Namekians body as a base, this Namek gained tremendous potential,to the level of Super Saiyan 2 vegeto. Making Gast one of the three strongest beings in existence across all the universes.
Super Namekians can reach incredible levels of power, but they still have limits, each fusion removes that limit tremendously, making piccolo reach perfect cells level after 37 years of training. But apparently hitting a plateau, while Gast went from king Cold's level to Super Saiyan 2 vegetto in 32 years.
Dragon Ball Multiverse
In Universe 16 and Universe 18 (and several other similar Universes), Piccolo attains his Super Namek form upon fusing with Kami to become the Nameless Namek - though he is even stronger than he originally was due to also having fused with Nail prior. Gast Carcolh of Universe 7 became an incredibly powerful Super Namek due to being the fusion of all of the Namekians on Planet Namek into one. King Piccolo of Universe 3 became a Super Namek as was stated in the novel. Becoming the nameless Namekian again.