What is dry insight?
Dry insight or bare insight (suddha-vipassana) is the 'direct' way (Pali: ekayano maggo) to insight (nibbana), without jhana meditation practice (i.e. without 'upacara samadhi' or 'appana samadhi'). 

This direct 'momentary concentration' is called in Pali parikamma samadhi. This is the tradition of Mahasi Sayadaw; U Ba Khin and S.N. Goenka and others. The practitioners are called 'bare insighters' or 'dry' insighters.

You can find more information in 'The Progress of Insight (Visuddhinana-katha)' by Mahasi Sayadaw (1994): 'This approach to the ultimate goal of Buddhist meditation is called ‘bare insight’ because insight into the three characteristics of existence is made use of exclusively here, dispensing with the prior development of full concentrative absorption (jhana).’