It's what everyone wants,  and what all monitor manufacturers claim they have, so what is it exactly. In a short sentence it's simply the ability of the monitor to produce a Sharp, Crisp picture.

So what all affects focus is more the main focus here,  no pun intended.

Dot Pitch,  Type of Grill - Aperture or Conventional,  Phosphor type, Type of impurities used to coat each cathode (an element of the electron gun), Hi-Voltage used for screen acceleration, and monitor Bandwidth, These all contribute to overall focus.  However for purposes of this section. I will deal with the basics. These being Dot Pitch and the focus grids or elements of the electron gun.

Dot Pitch is how close the spacing between same color red, green, or blue phosphor dots on the screen. A CRT's screen  is coated on the inside with a matrix of thousands of tiny phosphor dots. Each dot consists of three specks of colored phosphor: one red, one green, one blue. These groups of three phosphors ( Triads )  make up what is known as a single pixel. The closer the spacing, the smaller the pitch, meaning a crisper image will be able to be produced.

Focus Grids.  Most, not all, conventional CRT's utilize one focus grid, while Aperture Grill CRT's utilize two.  This allows for the electron beam to be focused on two planes, the horizontal and the vertical, rather than one.  More control generally equals a better picture.

Finally, there is Dynamic Focus, or DBF, Dynamic Beam Forming. Higher end monitors have added some special circuitry to allow for even more control of focus over more areas of the screen, rather than the more generalized which approximates over the entire screen area.  With DBF, the pixel shape is corrected to eliminate elliptical distortion at the edges, such as is found with out this feature, especially with conventional CRT's.