What is S in file permission -rwSr–r–?

Capital S in the user field means that setuid bit is set, but execute permission isn't (which makes little sense). Lowercase s means that both setuid bit and execute permission are set.

As for numeric values, it is everything explained in man chmod. Here's the excerpt:

A numeric mode is from one to  four  octal  digits  (0-7),  derived  by
adding up the bits with values 4, 2, and 1.  Omitted digits are assumed
to be leading zeros.  The first digit selects the set user ID  (4)  and
set group ID (2) and restricted deletion or sticky (1) attributes.  The
second digit selects permissions for the user who owns the  file:  read
(4),  write  (2),  and  execute  (1); the third selects permissions for
other users in the file's group, with the same values; and  the  fourth
for other users not in the file's group, with the same values.