Central or Middle Russian dialects is one of the main groups of the Russian dialects. Of Northern Russian origin, it has nonetheless assumed many Southern Russian features.
The official dialect (Standard Russian) originates from a dialect from this group.
The territory of the primary formation (e.g. that consist of "Old" Russia of the 16th century before Eastern conquests by Ivan IV) is fully or partially modern regions (oblasts): Moscow, Tver, Vladimir, Ivanovo, Pskov, Novgorod, Leningrad, Nizhny Novgorod, Yaroslavl (in Rostov), Ryazan (in Kasimov) and the enclave of Chukhloma.
The territory of the second formation (e.g. where Russians settled after the 16th century) consist of most the land to the South-East of Moscow, that is the middle and lower Volga, Ural as well as Siberia and Far East. It also includes Saint-Petersburg, whose dialect is fairly close to Standard Russian.
Central Russian dialects Wikipedia
Central Russian is a transitional stage between the North and the South, so some of its dialects closer to the North have northern features, and those closer to the South have the southern ones.