This a simple but effective device that I first picked up from James Newton Howard's score to "King Kong". It's very common though and I had probably heard something similar hundreds of times in other scores.
The basic idea is a string pad that crescendo/diminuendo's in support of a melody. As you might suspect it's very flexible harmonically and can also be used with woodwinds or brass instead of strings.
The ostinato helps establish a clear beat since it can be a bit obscure when the pad "fades" in and out. Harmonically it's Fmaj7-Gsus4-G for the first two bars and then Fmaj7-Gsus4. Leaving it unresolved gives a feeling of "what comes next" which can be useful in the right context.