A Slow is a mechanic that impacts a Hero's movement speed. They are percentage based, and are based on the enemy Hero's current movement speed.
For example, if you had 700 Base MoveSpeed and an effect granting you +10%, your new MoveSpeed would be 770. If you got another 10%, it would go up by only +70 and not by +77 because it’s still calculated from your Base MoveSpeed. Additionally, if you were Slowed by 20%, it would be a -140 regardless of your current actual MoveSpeed as Slows are taken from your Base MoveSpeed as well.
On the topic of Slows: unlike MoveSpeed Buffs, they do not stack. If someone Slows you by 20% and someone else applies a Slow by 10%, you will be Slowed by only 20% as the greater value is the one that applies. That said, the game does track Slow durations. Meaning if the 20% Slow is for 2 seconds and the 10% Slow is for 4 seconds, you will Slowed by 20% for 2 seconds and then by 10% for the remaining 2 seconds after the first Slow wears off.
It is impossible to Slow someone so much they cannot move. The minimum MoveSpeed in Paragon is 150, and the maximum is 1000.