Scouts are recently initiated Astartes, in their early stages of training.
In game Edit
Scouts are pretty much the first unit you will produce as they are vital for capturing strategic points early on in game. Scouts are quick and can infiltrate which is vital for them to carry out their role of.. well scouting. Scouts start out armed with a bolt pistol and combat knife. They can be reinforced to a squad size of four and have 3 weapon upgrades: bolter, shotgun and sniper rifle. They can have up to 2 weapon upgrades but can be increased to 4 with the Heavy Weapon Increase research. They are produced in the Stronghold.
When you've researched Tier 1 it is unlikely you will need scouts from that point but it's always a good idea to keep a few around because they can guard points and even assassinate enemy commanders if you research infiltrate and give them sniper rifles.