Well, it stands for "Pastry Chef Load Letter". It means that the "pastry chef", which is computer engineer slang for the printer controller, that is, the guy who controls the printer, that is, you, needs to load more letters into the printer's print head, because it is running low on letters and it needs a fresh load of letters so it can keep printing words, phrases, clauses, sentences, paragraphs, etc. So when you, the pastry chef, see "PC LOAD LETTER" flashing on your printer, open it up and dump in some letters. They can be scraped off the daily newspaper, or you can use a can of alphabet soup or a box of Alpha-Bits, or as a last resort, toy magnetic fridge letters or children's alphabet blocks may be used.
So there you go! That's what the fuck it means.