The Quick Selection tool selects an object by analyzing colors of neighboring pixels. Draw with this brush over the image, and the selection expands, automatically finding edges and filling areas.
New . In this mode a new selection will be created. If there are selected areas on the image, they will be deselected. Once you draw with the brush, the mode will automatically switch to Add.
Add . The selection area will be increased by adding new fragments.
Subtract . You can remove the areas of the selection you don't want by activating this mode.
Size. It's the diameter of the brush.
All similar colors will be selected, considering the edges of the objects and the value of the Tolerance parameter.
Tolerance. When the value of this parameter is increased, more colors are included in the selected area. If it is set to 0 the brush selects only the areas where it's applied.
For example, use the second command to remove the background: