The player filter is followed by either a single quoted string, or the name of a color followed by a quoted string, e.g.
    player "Ivanchuk"
    player black "Carlsen"
    player white "Kasparyan"

The player filter matches a game whose player field in its PGN header contains the given string.

If player is followed by white then only the white player is considered.

If player is followed by black then only the black player is considered.

The player filter can also be specfied on the command line by using the -player playername option followed by the name of a player. Use -white playername or -black playername to specify names of specified player colors from the command line

As illustrated in the gameinfo.cql, the player filter works with flipcolor and can be used to find wins by a player:

  {player white "Ivanchuk"
   result 1-0}
will find games that Ivanchuk wins (because flipcolor transforms both the player and the result, as they are both enclosed in a {...}.