The select tool allows you to select an already existing item so that you can perform further actions on that item.
When the select tool is selected, hovering the mouse over elements in the currently selected layer will show an outline of those elements. Clicking will then select that element. Once selected, any applicable property panels will appear, and will display the current options for that element. Changing any of those options will then change the selected element.
You can hold down the Ctrl key to allow you to select multiple elements from the current layer. Once multiple items are selected, moving will move them all as a group and resizing will also resize all the selected items. If you hold down the Alt key while you have multiple elements selected, you can remove items from that selection by clicking on them.You can also use Ctrl + D key to deselect or remove current selection.
The Layer List can also be used to select items.
You can also delete selected items, move them around, resize them, and copy/cut them.