Basically High Dots Per Inch. Pixel density.
For example a 2k and a 4k monitor might both be the same size. Say 26 inches.
A 4k monitor will have twice as many pixels as a 2k monitor in the same distance.
Usually in monitor specs, it will tell you the dpi. (Dots per inch).
I notice not all DE's support hiDPI, LxQt for example doesn't.
Comparison of Linux and Unix Desktop Environments
eylenburg.github.io
You can always make you resolution lower, which will make things like your fonts and icons larger (and sometiime fuzzier
to look at). But you can never make your resolution higher than your monitor supports.
The two common methods of doing this are fractional scaling and 2x scaling.
2x usually "looks" better because it uses the pixel next door. Fractional is a mathematical method
that "pretends" to borrow the pixel next via shading a corner of it.
Not all DE's support 2x scaling. One problem if you are using Xorg instead of Wayland, is that if you have multiple
monitors and you change the scaling in one monitor, it will affect the scaling in the other monitor.
This doesn't happen with Wayland.