The large purple rectangle is a protein channel across a cell membrane. It's normally closed, but can be opened by binding to a specific small molecule, which is called a ligand. Different channels have different ligands, and their presence/absence controls whether the channels will open.

In the cartoon Larry the ligand has been traveling with his friends, who represent the cargo to be transferred through that channel. When they reach the closed channel, Larry uses himself as the "key" to open the channel. Since channels usually only stay open while the ligand is still bound, Larry stays behind to keep the channel open while his friends make their way forward.

The joke would have worked a lot better if Larry's friends were different shapes. There's no visual reason why Larry is the only one that can act as the key.