The resolution of the monitor refers to the maximum amount of pixels that can be displayed on the screen. HD is a common standard referred to by television manufacturers which now generally use 1080p, as resolution that is made up by a horizontal display of 1920 pixels and a vertical display of 1080 pixels.

The relationship between screen size and screen resolution is referred to as pixel density, measured in Pixels per Inch (PPI). As the screen sizes become smaller and/or the number of pixels increase the visibility of individual pixels reduces resulting in a higher PPI.
Common monitor resolutions are;

1920x1080 - 1080p (HD)

This is one of the most common resolutions used currently, it’s commonly found in televisions & is supported by the common connectivity type HDMI, a modern, digital cable type commonly found in most households now.

2560x1440 - 1440p (WQHD)

A ‘Wide Quad HD’ monitor is a great step up from 1080p. A screen with such a high resolution will make photos look incredible & will give you much more workspace on your desktop. The term Quad HD implies a resolution of four times that of standard HD  or 1280x720.

4096x2160 - 4K (UHD)

4K is the next big step up, and is now available on quite a lot of televisions. As with TVs, a 4K computer monitor has a display resolution of 3840×2160. This is about four times as many pixels as a typical 1920×1080, or 1080p, display.