Board Thread:Questions and Answers/@comment-97.123.131.49-20170120103119/@comment-26599825-20170120163211

Mass Lock is a factor that affects your FSD drive.

You probably noticed that you need to be a certain distance away from a station in order to charge your FSD.

That also applies to being close to other ships, in a much smaller scale. In a nutshell, other ships have an influence on your FSD if they are nearby. They can slow down the FSD when it charges, increasing the time it takes until you can jump (rather than denying a jump completely, like a station). The Mass Lock Factor of a ship determines how much it will slow down other ship's FSD's. That factor is obviously higher for larger ships that have more mass. (Imperial Cutter's Mass Lock Factor: 27; Adder's Mass Lock Factor: 7)

See http://elite-dangerous.wikia.com/wiki/Hyperspace#Gameplay for more details.