Repulsion Device

Forcefields or repulsion fields are energy-based barricades found in technologically advanced areas of the wasteland, such as the Mariposa Military Base, the underground base beneath the Cathedral or the Enclave's Raven Rock base in the Capital Wasteland.

Types of force fields
It is impossible to pass through a yellow-colored force field without disabling it. It is, however, possible to pass through a red-colored force field, but this will result in varying amounts of HP damage. Dogmeat is said to have succumb to death by attempting to pass through a force field in the mutant military base in The Vault Dweller's Memoirs.

Disabling force fields
In Fallout and Fallout 2, yellow force fields may be temporarily disabled by utilizing the "Repair" command on the force field emitter.

In Fallout 3, repulsion fields can mostly be disabled using nearby terminals, which requires a Science check.

Fallout 3
Repulsion Fields are encountered at 3 locations:
 * Raven Rock: These can be disabled by using a nearby terminal.
 * Around the Jefferson Memorial, only from The Waters of Life till Take It Back!. These are disabled by Liberty Prime.
 * At the Mobile Base Crawler. The following can be done with them by using the nearby repulsion field control panels:
 * Do Nothing: Nothing will happen
 * Turn off the repulsion field: Requires 80 Science, and will completely turn the field off.
 * Set a timed charge: Requires 50 Explosives, this will blow the panel up and partially disable the field. If you then  walk through the field you will take damage.
 * Smash the Control Panel: this will destroy the panel, causing you to take damage and will partially disable the field. If you then walk through the shield you will take damage.