With this tool, you select UV points/polygons by dragging a lasso around the polygons. To complete the selection, release the mouse button. The two ends of the loop are joined automatically.
In the Active Tool manager, if Tolerant is enabled all UV points/polygons that are inside the marquee or intersected by the marquee are selected; If Tolerant is disabled, only the UV points/polygons that are fully enclosed by the marquee are selected.
If Only Select Visible Elements is enabled, you can only select elements which are visible, i.e., not behind other elements. For example, you can select the polygons at the front a sphere but not the polygons at the back.
See Rectangle Selection.