This command toggles 'noclip' mode. In noclip mode your character has collision disabled and will be able to fly through walls, floors, and any other solid object that would usually not allow for this. This command will not work if you have a target selected - you must deselect your current target in order for this command to work.
This is the only way you can execute the tcl command. The above command will enable or disable collisions for your character. This is not a target command - if this command isn't working for you, it is probably because you have a target selected.
ToggleCollision is simply a longer version (alias) of the TCL command - it has the exact same function.