The metaphor here is from cooking on a stove with several burners of varying heat…food cooking at a lower temperature on a back burner receives or requires less frequent attention than that cooking at a high temperature on a front burner. Compare with the mainly North American expression cook on the front burner meaning be on the way to rapid success.
RELATED IDIOMS :
front of house
the parts of a theatre in front of the proscenium arch
the business of a theatre that concerns the audience such as ticket sales