A Selection is a set of highlighted blocks that affect the functionality of other systems.
There are three selection tools in Axiom that you can use to make selections. Each of these tools feature several options to further finetune what and how you might want to select something. See Selection Tools for more info.
With a selection, you can constrain the brush strokes of Tools (note: some more complex tools don't respect selections yet, this will be addressed soon).
You can also apply Operations to the blocks in your selection.
Finally, to clear a selection, use 'Select > Clear' or press the Enter key