This is the process of cleaning the inside of your engine by removing sludge, deposits and other things that have contaminated your engine. This is done by using a chemical additive (like a detergent).
You may want to consider a motor/engine flush if your vehicle has never had one and you are over 100,000 miles. You can perform this at every oil change if desired- it just costs a little extra for the detergent and takes a little longer.
If you are using a gasket re-sealer in your engine oil, you may want to abstain from a motor flush.