Primitive Sponge, also incorrectly known as SpongeGar, is a dank meme that originated on 4chan in 2014 and gained popularity in 2016. The meme depicts Primitive Sponge from the 1999 SpongeBob SquarePants episode "SB-129" in an alarmed pose. Its main meme rival is Dat Boi.
Please see New York Magazine's usage guide for Primitive Sponge memes if you do not understand when to use a Primitive Sponge meme. There is also a Daily Dot article on Primitive Sponge if you prefer.