mnemonic
Explanation
This comic jokes about mnemonic devices -- memory tricks used to remember information. A teacher says "There's an easy trick to remember this: Apple." She then explains that the "A" is for "Accretion Disk" -- referring to the disk of gas and dust that spirals around a forming star or black hole.
The humor is that mnemonic devices are supposed to use familiar, easy-to-remember words to help recall difficult concepts. But here, the "mnemonic" goes in reverse: the familiar word "Apple" is being used as a mnemonic for the obscure astrophysics term "accretion disk." The caption -- "Mnemonics are much easier when you're talking about the early solar system" -- adds the punchline: when your subject matter literally starts with the formation of the solar system, everyday words like "apple" happen to share initials with astrophysical phenomena, making mnemonics trivially easy (at least for the first letter). Of course, knowing that "A" stands for "accretion disk" is hardly a useful mnemonic since the hard part is remembering the concept, not the letter it starts with.