@creativejuices – Cutting in “is what you do when you want to keep the paint off the abutting surface, i.e.: ceilings, casings, moldings or… window mullions and sashes.”
In other words, you want to paint the wall blue but leave the ceiling and door trim white. Instead of running painter’s tape along all the edges to make sure the blue paint stays where you want it, you learn how to lay a straight line of paint along the edges of the to-be-painted area without slopping any over onto the bits you want left white. It saves a huge amount of time, and makes the difference between a professional quality job and a sloppy one. Thus, it also saves money since you can do it yourself.
I learned how to do it when I was working as a curator in a small museum and everyone got involved when a big new exhibit was to be installed.