In simple terms UT1 is the time if we let seconds be defined by the earth's rotation. UTC was kept in sync with this rotation by introducing leap seconds, while still adhering to the stable ticking of TAI (cesium clocks).
From now on UTC will in practice be TAI but with a constant offset (=all previous leap seconds).
From now on UTC will in practice be TAI but with a constant offset (=all previous leap seconds).