There are certain ideas that are so commonly used in film scripts that they have abbreviations. Some of these are used as Extensions, like O.S. and V.O. Some of these are used in page breaks or when Action splits Dialogue, like CONT'D.

Others show up in Action. The most familiar are "b.g." -- background, "f.g." -- foreground. For example:

John mows the lawn while, in the b.g., a plane crashes. 
Runners cross the finish line as bushes in the f.g. 
start to shake and twitch.

Should you use them? Your call. I find them distracting and think that a script is easier to read without them. But if it's important to communicate foreground or background (don't forget "don't direct"), use them as necessary. Some writers put them uppercase. Others use them lowercase. Again, it's up to you.

Another common abbreviation is used in Shots. It's POV, for point of view. As if we're looking out the eyes of a character. No periods in POV.

          Marty, where are you?
Looking through the giant carpet fibers at Bob 
approaching the 20-foot shoe!

Another one that you'll see every now and then is MOS. MOS means silent, without sound ("mit out zound" as the old German directors would say... some claim this is the origin of . I've never used MOS; it's really one of those cues that's there for the production staff. If the sound guy sees a day's worth of MOS scenes, he knows he's got the day off.