what is the num value of "s" in -rwsr-x--- (chmod)?

[john smith]

what is the "s" letter stand for? r stands for read access,w write, and x 
access files and subdirectories, I can even c a "d" i.e.
drwxr-xr-x. what does those stand for and what are their numerical access 
mode values? to further add to my question, how do I give a user access so 
that he/she will be able to use the pppd/pon command?
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