What did the united states and the soviet union agree to do in start?

The Strategic  Arms Reduction Treaty (START) number one was designed to
reduce the number of nuclear weapons.  It was signed July 31, 1991 and went into full force by 1994.  Eventually it was to control and remove warheads and nuclear weapons.  Start 1 achieved an 80% removal which was considered to be a success.  Other treaties followed.