Heat island Effect is the term used to describe the hotter/elevated temperature areas in urban areas or cities. This means that areas in the cities are warmer than rural areas because of the modifications and alterations in/around the land/area. For this reason the area has a few degrees higher temperature. Some of these modifications include asphalt, roads, cars, pollution in the streets, etc.
The answer for your second question is that we try to plant trees and vegetation and reduce the footprints of asphalted oarkings and roads, use pervious pavement, install green roofs on top of buildings, decrease number of cars pollution, decrease GHGs etc.
I hope this brief explanation helps.
