Hello. Some programs written in 6502 Assembly have this reserved word ".res" which is followed by a value usually of "1,$00". What does this mean?

Depends on which variant you're talking about. It makes it a lot easier if you tell us which tool you're talking about, because there is no standard for assembler directives.
That looks to me like the ca65 directive for reserving an area of memory(in this case 1 byte), using a fill value of $00. But since I don't know what tool that source is intended for, I can't be certain.

If it's ca65, then: https://cc65.github.io/doc/ca65.html#ss11.93

Cheers,

-Gordon

_________________
--
Gordon Henderson.
See my Ruby 6502 and 65816 SBC projects here: https://projects.drogon.net/ruby/