"A metro mayor is the directly elected leader of a combined authority.  Combined authorities are statutory bodies made up neighbouring local authorities that broadly cover a city-region. These authorities agree to work together formally to pool resources and powers to function more effectively on issues such as skills or transport. The metro mayor chairs a cabinet of the leaders of combined authority councils.

Once in place, metro mayors have significant executive powers and funding available to them to make strategic decisions across whole city regions. They also sit as the chair of the combined authority board, made up of the leaders of constituent councils and which authorises the use of other powers and funding unlocked by devolution deals."

https://www.centreforcities.org/publication/everything-need-know-metro-mayors/