In most real journeys the speed does not stay the same. A car speeds up, slows down and stops at traffic lights. Motion like this, where the speed keeps changing, is called non-uniform motion. We cannot describe it with a single steady speed, so we use the average speed instead.
Average speed is the total distance travelled divided by the total time taken: average speed = distance ÷ time. Distance is measured in metres (m), time in seconds (s), so speed is in metres per second (m/s). It is a mean value for the whole journey. At any moment the object may have been going faster or slower than the average.
Any time spent stopped is included in the total time. For example, a cyclist rides 600 m in 100 s and then 400 m in 200 s. The total distance is 1000 m and the total time is 300 s, so the average speed is 1000 ÷ 300 = 3.3 m/s. Do not add the two speeds and halve them, because the two parts of the journey took different lengths of time.
If distances are in kilometres or times in minutes, convert to metres and seconds first (1 km = 1000 m, 1 minute = 60 s). In a practical, you can roll a trolley down a ramp, measure the distance with a ruler and the time with a stopwatch, then calculate its average speed.