The Infiniplex is a very large building (about three times the size of The Seven Seas) that is green in color. There are also images of Mermaid Man and Barnacle Boy on the front side of it, possibly to promote their new movie.
The Infiniplex was first seen after Sandy, Patrick, and Mr. Krabs all ran into SpongeBob asking if he was going to see the movie, Mermaid Man & Barnacle Boy: The Dark Tide Rises. SpongeBob is eager to see the movie but wasn't able to get in at the time due to him wearing long pants. Mr. Krabs eventually came out due to the crying and cuts off SpongeBob's pants (and legs) turning them short again.
It is a parody of a multiplex, a movie theatre complex that has multiple screens within a single complex. Due to its name, it can be assumed that it has an infinite (or a really large amount) of screens inside it.
This is supported due to its large size, especially when compared to The Seven Seas cinema next to it.