A base of fire is a military term for a supporting force that provides overwatch

Overwatch

In modern warfare, overwatch is the state of one small unit or military vehicle supporting another unit, while they are executing fire and movement tactics. An overwatching, or supporting unit has taken a position where it can observe the terrain ahead, especially likely enemy positions. This...

 and covering fire to other advancing units while they are executing fire and movement

Fire and Movement

Fire and movement or fire and manoeuvre is a military tactic that uses suppressive fire, or threat thereof, to decrease the enemy's ability to return fire, organization and unit cohesion, and morale. The tactic is used by small unit commanders on the modern battlefield...

 tactics. A base of fire can be a platoon

Platoon

A platoon is a military unit typically composed of two to four sections or squads and containing 16 to 50 soldiers. Platoons are organized into a company, which typically consists of three, four or five platoons. A platoon is typically the smallest military unit led by a commissioned officer—the...

 during company fire and movement, by individual armoured fighting vehicle

Armoured fighting vehicle

An armoured fighting vehicle is a combat vehicle, protected by strong armour and armed with weapons. AFVs can be wheeled or tracked....

s (esp. tank

Tank

A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...

s) or infantry sections

Section (military unit)

A section is a small military unit in some armies. In many armies, it is a squad of seven to twelve soldiers. However in France and armies based on the French model, it is the sub-division of a company .-Australian Army:...

, in platoon fire and movement, or even by fireteam

Fireteam

A fireteam is a small military unit of infantry. It is the smallest unit in the militaries that use it and is the primary unit upon which infantry organization is based in the British Army, Royal Air Force Regiment, Royal Marines, United States Army, United States Marine Corps, United States Air...

s or individual soldiers, in the final stages of an assault.