Mecha is green tea made of tea leaf buds. These buds are separated from the tea leaves during processing of Gyokuro or Premium Sencha, therefore Mecha has comparable quality although its price is much lower.
Mecha has strong aroma and flavor. Because of this it can be re-steeped for many times while maintaining the flavour. But this advantage of Mecha is also its disadvantage, because of the strong flavour, Mecha could taste bitter if over-steeped. Tea buds has the concentrate of the nutrients, so you will get higher level of Catechins or L-Theanine and Caffeine.