to move information between one's memory and some other source of information, e.g. a book. (These respectively correspond to "fast-access memory" and "slower storage" referred to in the "origin" section below.)
To keep something "swapped in" is to keep it fresh in one's memory. To "swap something out" is to write it down before you forget. To "have swapped something out" is to have forgotten it because - in theory - some other information overwrote it in one's brain.
I read the programming manual every few months to keep it swapped in.
Person, being interrupted while writing: Just a second, let me finish swapping this out.
Person A: Which order do the functional parameters go?
Person B: I swapped that out. Let me check the manual.
In computer speak, to move information from fast-access memory to a slower storage device, e.g. disk ("swap out") or the other way around ("swap in.") This often refers to loading data from disk into memory as needed (swapping in) and writing the data back to disk when it is no longer needed (swapping out.)