How to get rid of this weird grid that appears when I resize a brush?

[TF2CutContent]

-snip, quitting-

 

[saph]

It's in the top left of the screen. Second button from the left that has a small grid and says 3D.

 

[Micnax]

It's a feature to help you resize objects in 3D space rather than just 2D space. The shortcut for it is X, which you might've pressed by accident instead of Z (which is used for 3D camera movement)