Here is the text from the help file on the selector tool...

Because the Selector tool is the most used tool, there is an additional shortcut that lets you switch into the Selector tool to perform a selection or other operation, and quickly switch back to the previous tool. If you are using another tool, you can easily switch to the Selector tool:

Press and release Alt+S to switch to the Selector tool. Press again to return to the previous tool.
Or
Hold down Alt+S to select the Selector tool. Perform the required operation while the keys are held down, then release to return to the previous tool.