They are the minimum / maximum values that can be stored in variables of the respective type.
Short: -32768 to 32767
Long: -2147483648 to 2147483647
Float: -3.4*10**38 to 3.4*10**38
If in either direction you exceed the listed values, the variable will "wrap" around to the other end.
set var to 32767
MessageBox "var = %g" var
; shows "var = 32767"
set var to ( var + 1 )
MessageBox "var = %g" var
; shows "var = -32768"
Note: If you want fractions then you must use floats. Storing "1.5" into a long or short would just result in "1" (everything after the decimal is truncated).
Basically to store an integer like 1 in a computer, you need all bunch of ram. Think of it like this:
long x = 1
"0000000001"
But if you know the limits of your variable, that it will only track to max 30 for example then you can optimize it like this:
short x = 1
"00001"
Then there is floats, when you know you will need fractions.
So select the best fit for each variable.
Try studying some https://www.scribd.com/doc/7007668/Morrowind-Scripting-for-Dummies-9 script examples/explanation. If you still can't understand variables, scripting is not for you
[EDIT]You can still try to copy/paste/assemble already existing/working script snippets from here and there, but this will not work much if you don't really understand the logic
[EDIT2]to ease studying scripts in Morrowind.esm, you could try right click Morrowind.esm in Mash, export\scripts so you have a unique .txt file with all the scripts and see how variables are used everywhere at a glance