If you are working on a certain object(s) and find yourself accidentally deselecting the object(s) or selecting other objects in the viewport, you can use the Lock Selection button to prevent that from happening. Hit the Spacebar to lock/unlock the currently selected object and you will not be able to deselect that object or select any other objects until you unlock the selection. This is good to be aware of anyway, as many beginners hit the spacebar accidentally and then don’t know why they are stuck in a selected object.