Hilight the required media by selecting their thumbnails with the mouse then click the select icon to reduce the media view to this new selection.

Thumbnails can be selected in a number of ways using the mouse:

Single click - click a thumbnail to highlight that one only. Click between thumbnails to highlight none.

Command click - holding down command while clicking a thumbnail will add that thumbnail to the selection. If it is already highlighted, it will be removed from the selection.

Shift click - this will select all the thumbnails between the highest or lowest highlighted thumbnail, and the thumbnail just clicked.

Marquee selection - by dragging the mouse over the thumbnails you can select many at once. The command key modifier works in this mode too.