After x raying my teeth the dentist found what he called deep lesions right inside my teeth and all across my mouth. What are teeth lesions What casues them and what is the treatment?

In this context, 'lesion' simply means 'cavity'. Apparently, you have many cavities in several teeth throughout your mouth. They are quite extensive, and some may even require root canal therapy in addition to fillings or crowns.

Cavities are caused by acids produced by bacteria exposed to sugars over a period of time that break down the enamel of the teeth. Once the enamel layer is breached, the decay quickly extends into the material called dentin. If not treated properly, the decay can reach the pulp of the tooth where the nerve is located. If that happens, the tooth will die. To save the tooth, a procedure commonly called a root canal must be performed. Otherwise the infection will cause great pain, and the only alternative is to remove the tooth.

If you doubt the dentist's diagnosis, ask for copies of the x-ray films and seek a second opinion from another dentist.