That's when swimmers get into starting position on the block and push off after a signal. There are two ways to position your feet on the starting block: both legs on the edge of the block (grab start) or one leg in front and another leg in the back (track start).
Today, most swimmers do a track start as it's the most effective way to dive forward.
It gives swimmers a better balance and weight distribution on the block and allows them to get into a streamlined position much quicker, diving into the water with a clean entry. Additionally, track start has a lower reaction time, meaning that the swimmer's feet leave the block faster than in grab start. Lastly, the track start position allows swimmers to dive with greater power by pushing off with both hands and legs.