What are “grid hotkeys”? I saw them in the preferences. What are they? How do they work?
Basically, it makes it so all units in a building, unit commands, etc. are assigned hotkeys based on their position in the UI. For example, since a Marine is the Top-Left in the Barracks, its hotkey would be Q. Here's a page I found that has a small picture to illustrate: Grid Hotkeys.
Grid keys replace the default keybinds by actually mapping the squares of the unit cards to certain buttons on your keyboard.
I don't know the exact commands off hand, but it works something like this:
QWERT ASDFG ZXCVB
These keys become the 3x5 grid of the unit card. Pressing "T" triggers the top rightmost command, usually attack move, no matter the unit, and instead of the usual "A" in the default keybindings.
Grid Hotkeys are a hold over from Starcraft 1.
Default Hotkeys use letters in the building/action command for which reference. For example, to build a Bunker you can use the U key.
Grid is an alternative style that mimics the layout style of commands on the keyboard. For example the Stop command is normally bound to the S; grid instead binds it to the W key.