Negan is a Bruiser hero, with the following Leader Trait:

At the beginning of every turn, Negan marks the closest enemy for 1 turn. Marked enemies receive more damage, deal less damage, and they cannot inflict status effects.
As a leader, Negan marks 3 enemies instead of 1.

For example, if a Tank walker is marked, it will not be able to stun your survivors. In a similar manner, if a Spiked walkers is marked, its attacks won't cause bleeding.
It is important to note that this applies to status effects that are directly inflicted by the enemies. If a burning walker is marked, it will still deal fire damage, as it is not inflicting a status effect; if a marked Goo walker is exploded, it will still stun your survivors.