When the temple was consecrated, God sent fire down from Heaven to light the fire for the altar of burnt offerings. The priests were to keep that fire burning all the time and any fire for sacrifice or for burning incense in the temple was to be taken from that fire God had initially started.
Aaron’s sons Nadab and Abihu got fire from their own source, strange fire that God had not ordained and they were destroyed by fire from Heaven. Nadab and Abihu had the idea that they would worship God in their own way, but God wants us to do things His way - with our whole heart, in spirit and in truth.