This is the Select tool. Clicking on this allows your mouse pointer to be used to select atoms and bonds within your chemical sample window. Selection of atoms and bonds can be accomplished in a number of ways.

If you wish to select a single atom or bond, just click on the Select tool and then click on the desired atom or bond.

You can select all atoms and bonds in a rectangular area by putting the mouse pointer at one corner of a rectangle, click and hold the left mouse-button, drag the mouse pointer to the opposite corner of the rectangle and then release the mouse-button. All atoms and bonds contained in this rectangle will become selected and show up highlighted.

Alternatively, you can select atoms and bonds one at a time, adding to the list of selected atoms by holding down the Shift button and clicking the mouse pointer on each of the desired atoms and bonds.

Once you have selected atoms or bonds you can move them by clicking and holding the mouse-button and then dragging them to their new location and then releasing the mouse-button. The modified position does not show up until you release the mouse button.