Movement speed is a modifier to the base speed a character can run.
Chest armour and shields induce movement speed penalties when equipped. The exact penalty can differ depending on the base item type, but in general strength armour has greater penalties than dexterity and intelligence armour.
There is no cap on movement speed.
Movement speed is beneficial for characters that employ evasion as the primary form of damage reduction, since it requires a play style where physically avoiding enemy spells and attacks is often necessary.
In the case of Armour Master, the movement penalties from equipped armour are reduced to zero. The effects from this passive are most noticeable with strength armour equipped.