This command ignites your character (setting you on fire), but you do not take damage. You can set fire to other flammable objects/entities by colliding with them (e.g. jumping on them).
The syntax for the burn command is as follows:
burn Default Value:
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Full listFind below working examples of the burn command.
burn
This is the only way to use the burn command. Typing this into the console will set you on fire.