The browning phase is self explanatory - it is where the beans turn from green to brown. Aromas are getting sweeter but still have hay/grassy notes, similar to young sweetcorn from the fields.
The Maillard reaction also happens during the browning phase. The development of flavours and sweetness happen mostly within this important phase.
A steady, continuous roast curve is very important to get the best out of your beans. Stalling your roast curve will lead to roasty tastes which are not desired. The browning phase ends at first crack.