Command_Slap is the callback for the command sm_slap, when a client invokes the command through chat/console his/her client id is passed to client and args contains the number of arguments he/she passed. The Command_Slap code block is then run to see what actions should happen. The arguments he/she passed can be retrieved in the form of a string with GetCmdArg. MAX_TARGET_LENGTH is defined in one of the includes the plugin has used to compile. If you look at sourcemod.inc you'll see that it includes other includes, this just creates a large tree of includes. There are many things that are defined with simply including sourcemod.inc. Some examples: NULL_VECTOR PLATFORM_MAX_PATH MAX_NAME_LENGTH
If you simply want to print stuff to the client, look at the various PrintTo... functions in the api.
Api here
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