Very often the guitar is used as an accompaniment instrument - say a singer who sings a song and accompanying himself or herself by strumming the guitar. As such the guitar is popularly used is sing-along sessions.

There are actually other ways of playing the guitar as accompaniment to songs:-
(1) fingerstlye picking
(2) plectrum picking
(3) strumming - sweeping 3 or more strings together to play chords using fingers or plectrum.
(4) combination of any one or more of the above.

Here we are concerned with (3) above - strumming the guitar to accompany the songs we sing. We sweep or brush 3 or more strings at any one time in a certain rhythmn and tempo.

In fact we have already done this in this lesson here and here.
We learn 3 chords, strum gently by stroking downwards while we sing.

We can also strum with a combination of down strokes and up strokes; sometimes strum the lower (bass) strings or upper (treble) strings with different accents to give the accompaniment more flavor.