Initially, as the rocket stands the acceleration should be zero, according to 1st Newton's law of motion, but the accelerometer shows non zero, pointing downwards, because of the gravity.

The idea is to detect the moment when the acceleration vector changes - when it goes upwards. But without a gyroscope it's impossible to do this, because we can't get rid of the gravity error. So here we need to have linear acceleration for this task and this can be calculated by having an accelerometer and a gyroscope.

Linear acceleration refers to the acceleration measured by our acceleroemeter but without gravity error vector.