Pull-down

The same idea as a pull-up with the other default. One end on the signal, the other on ground, so an undriven line settles low instead of high.

flowchart TB
    D["a driver,<br/>when it is driving"] --> S["the signal"]
    S --> IN["the input<br/>that reads it"]
    S --> R["10k"]
    R --> G(["GND"])

Which one a design wants is a question about the safe state, not about preference. A line that enables something usually gets a pull-down, so the thing stays off until firmware deliberately turns it on. A line that holds something in reset usually gets a pull-up, so the part stays held until something releases it. Getting the choice backwards gives a board that powers up doing the thing nobody asked for.

Both appear as strap resistors, where the level a pin reads at power-on selects an address or a boot mode.

Where the course teaches it: chapter 1 for the role, and chapter 4 for why an undefined level is the error case.