“Hair training” or “scalp training” is the act of “training” your hair to only need to be washed once a week or never. The thinking is that the additives in conventional shampoos dry out strands, so by cutting back on washing, your hair will get back to its natural, healthy self.
How often you wash your hair largely depends on your hair type and the amount of sebum your scalp naturally produces, which varies from person to person. For folks who wash their hair five times per week or more, Diaz-Santin recommends taking things slow. Try going down to four times a week, then gradually down to three times, then twice a week, and see how that works for you.