It's a form of easing.
Say that you want an object to travel to a destination 100 pixels from where it is at.
What you can do for example is have that object travel 60% of the distance remaining, and repeat that process until it is super close. This will create the visual effect of it slowing down on approach.
The calculations are something as follows: At position 0, going to 100, that's (100 - 0) * .6 = 60. Your new position is 0 (current) + 60 = 60. This calculation happens again, 60 to 100 is 40. This gap of 40 is multiplied by .6 as well, to give you another step of 24. 60 + 24 = 84.
You will continue to edge closer and closer to the end goal. At some point when the pixel movement is below a single pixel, it's best to stop and just set the final distance.
A video I would recommend watching is: https://www.youtube.com/watch?v=zLh0K1PdUbc