This is the 4th most frequent Japanese word.
を
The particle "を" indicates the direct object of a verb.
In this sentence, 'を' marks '映画' (movie) as the direct object of the verb '見ます' (watch).
私は映画を見ます。
I watch a movie.
In this usage, 'を' specifies '川' (river) as the location traversed by the action of the verb '渡ります' (cross).
川を渡ります。
I cross the river.
Here, 'を' indicates that '友達' (friend) is the direct object being acted upon by the verb '送ります' (take).