What does bishop do?

Bishops generally oversee a diocese, and tend to the clergy as well as the parishes within that diocese... in this sense, you can think of a bishop as a Catholic "district manager". There are some ceremonies that they carry out that priests don't usually do, such as confirmations; they also sometimes lead mass at the main church within their diocese
A Diocesan Bishop confirms, ordains Priests, administers a diocese, preaches the Holy Gospel, performs visitations of, monasteries, convents, and seminaries, instructs the faithful in sound doctrine, and performs other Priestly duties as well.

An auxiliary Bishop of a diocese does whatever work he is instructed to do by his diocesan ordinary (the Bishop of the Diocese).

Bishops directly in the service of the Holy See serve in the offices of the Roman Curia, oversee certain Apostolic works, or can be employed in diplomatic service, such as NunciOS of the Holy See to various countries.
A bishop is like a pastor or a preacher.