What is "Focus" in the context of " 2D geometry "
When working in 2d drawing mode, you can force solidworks to "focus" on a specific view, or alternatively, the drawing sheet itself.
When you have "locked view focus" on a view or "locked sheet focus", anything you sketch will be related to that view (or the sheet). You can test this by moving the view after placing some items (lines, annotations, etc) and see the items move with the view.
Sometimes when we forget to "lock view focus" or start sketching further away from the view, the items we're sketching become associated with a view other than the one we intended, and when we move our view, the items don't move with it and don't behave appropriately when we move the views around.
Items that don't want to be associated with any views should be placed with "lock sheet focus" checked so they associate with the sheet rather than the nearest view.