I'm no expert, but...
It's like, creating/editing/deleting things on the SA map I guess; for example, you can use a mapping program to construct a house.
'They' use programs to, such as
SA-MP Map editor -
MTA and etcetera.
And again here's when Youtube comes in handy, search there for tutorials -
MTA or
SA-MP Map editor.
Oh and
Ματηεδ also has a mapping showcase, I don't know if you saw it or not but here it is -
viewtopic.php?f=42&t=103 He has a lot of experience so I guess you could probably ask him for tips.