DLP is an initialism for Designable Linker Puzzle, coined by Foldit scientist beta_helix.
The Foldit blog post Coronavirus Designable Linker Puzzles describes this type of puzzle.
The idea of a DLP is to design a section of protein that links two separate binder proteins, which are designed separately. In a DLP, the binder sections are mostly locked or held in place by constraints, and only a "linker" section can be freely designed.
Puzzle 1912b: Designable Linker: Coronavirus Spike Binder is the first puzzle of this type. In this puzzle, the fixed binders and the designable linker section are a single chain, and there are also two smaller chains, which apparently represent the binder targets. Like the binders, the targets are also locked.