%ID% -- For the player ID

%ID_Name% -- For the player name

%ID_Health% -- For the player health value

%ID_Speedmod% -- For the player speed modifier value

Etc, you can use any of the options that are found on http://cs2d.com/help.php?luacat=all&luacmd=player#cmd

My map has 5 entities, 2 for the spawns, 2 buttons (named ButtonA and ButtonB) and 1 of the message type (named OnlyForPlayer_1) with the message "Player with ID 1 is here"

The ButtonA trigger field has this:

wthisC sv_msg2 %ID% Hello %ID_Name%!

The ButtonB trigger field has this:

wthisT OnlyForPlayer_%ID%!

There's a player with ID 1 named Yank and another one with ID 2 named Yaxis

When Yank presses the ButtonA, a message is sent to him saying "Hello Yank"
Same happens for Yaxis

When Yank presses the ButtonB a message is sent to all players saying "Player with ID 1 is here"
However when Yaxis does this it will not do anything because it will end up triggering the entity OnlyForPlayer_2 (since 2 is his ID and that entity does not exist)

This is a pretty simple explanation of what the script allows you to do