Have you ever seen this thing appear next to your cursor?
If so, you will have found that you can only select certain things - often not the thing you wanted to select!
So what is it?
How did it happen?
And how on Earth do you get rid of it!?!
It is a Selection Filter - there are (as standard) 3 shortcut keys that it is very easy to hit by mistake.
They are V, E, X.
Quite appropriate that they spell "vex" if you don't know what you've done!
V = Vertices,
E = Edges,
X = Faces.
(X is used for faces because F is used for "Fit to Screen")
It's quite easy to turn these filters on accidentally if you have tried to re-name something and not actually gone into renaming it - then you start typing and "Ooops! - What's that?!?!"
The easiest way to find out which one you've turned on (you can have them on in combination, which gets confusing) is to hit F5.
doesn't clear the filters, it's just a shortcut to show or hide the Filter Selection toolbar.
The default position for this is at the bottom edge of your screen.
If you look at the toolbar, you will be able to see which button(s) on it are pressed - click on those to turn the filters off.
There are, as you can see, filters for all sorts of things - surface bodies, solid bodies, axes, planes etc.
But it's only Vertices, Edges and Faces (X) that have shortcut keys as standard.
If you like using filters, it may be useful to know that F6 will toggle any filters off - or back on again if you hit it again.