Slow is a status effect that makes the victim move slower and have reduced jump height. Coco's Blaze has Slow right off the bat, and upgrades such as Ribbit Snail Slime and Tentacle Soup give other skills a slowing effect.
When two or more slows from different sources are applied to the same target they will not stack: the strongest effect will always be applied or overwrite the weaker one. Different upgrades from the same ability will still stack, however.
Slowing abilities and upgrades
Note: Yuri's Time Warp and Raelynn's Timerift are labeled as time modifiers and not considered "true" slowing effects.