What are the slashes with letters for – as seen in
set /p or
for /f etc?
They are "command line switches", and you use them to give a command more information about what you want it to do.
For example, the
dir command lists the files in the working directory, one file per line. If you say
dir /w then it lists them in "wide" format, with several files per line.
/w is a "command line switch".
To find out information about a particular switch, you need to read the documentation for the command it relates to. For
dir, you can type
dir /? - the
/? switch tells the command to give you help about how it works, including the switches that it accepts.
The two commands you mention,
for, also accept the