Many commandline tools take arguments. There are one-letter, short, arguments and ... long arguments. They change the default behaviour of such a tool.
Short arguments are prefixed with a single dash
Long arguments are prefixed with a double dash
Short arguments can be combined into
ls -l -t -r is the same as
Long arguments need to be distinguished from combined singles, and to do that, a long argument is prefixed with a double dash
ls -l -t --reverse or
ls -lt --reverse
Long arguments are easier to remember, short ones faster to type.
Most commands have a manual page which explains these arguments in detail. Eg.
man ls for the manual page of the ls command which I used in my examples above.