The estimator multiplies each quantity by its labor unit to get total hours. Hours times the burdened labor rate gives labor cost.
Example: an electrician applies one labor unit to each recessed light fixture and another to each foot of conduit.
Published labor units are a starting point. Your own job records are better. Adjust for the conditions of each job, such as work height, access, building size and schedule pressure. Track actual hours by cost code and compare them to the estimate after each job.