Suppresive Fire to me is laying down a stream of fire to a dedicated spot.
This can server many purposes for your team.

The role of suppressive fire to pin down an opponent.
The tactical maneuvering of your team mates may rest on you laying down a stream of lead.
So your opponents can hopefully pull back to a more secure position. Which is because of the fear of getting hit. Thus allowing your team to perform a tactic that would otherwise be more dangerous than it already is.

Your role is not essentially to kill the opponent. Though the incapacitation or the neutralizing of an enemy is not bad. The whole concept is to keep the enemy down and hopefully ignorant of the surroundings with your team that just happens to be in those surrounding.
So while you can expect a customer to pass through an area to attack or defend a target. You can also lay down some suppressive fire so your team can advance in a direction with as minimal resistance that can be achieved.


----
Now you may think how I can I apply this to CoD4.
Here are some examples that I use when trying to defend or attack a position.

If I see a smoke cloud bursting out in front of me.
I shoot directly into it. The enemy is trying to either fake a maneuver or actually use the smoke as cover. So if they are using the smoke as a cover. I can hopefully tag some customers as they run through it or hopefully make them think twice or three times about using that particular smoke as cover.

Area denial.
The thing about any game is that it's not real. The intent is to gain the maximum entertainment value out of a code structure. So with that said the field of play is not exactly as vast as the real world which is expressed as maps for CoD4.
With time we all become familiar with the ins and outs of a map.
Where to stand where not to stand and fire from.