Acceleration is the change in velocity divided by the time taken. It is measured in metres per second squared (m/s2). If something slows down, its acceleration is negative and we call it a deceleration.
You should be able to estimate the size of everyday accelerations. A car pulling away from traffic lights might go from rest to 30 m/s in about 15 s, which is an acceleration of about 2 m/s2. A car braking hard decelerates at about 8 m/s2.
An object falling freely near the Earth's surface accelerates at about 10 m/s2. This is about 5 times bigger than a car pulling away. Rough numbers are fine for an estimate, so round to one easy figure.