Beam shields are energy-based mecha shields. A beam shield projects a powerful energy field that disrupts and absorbs the damage from an incoming attack, working similarly to a physical shield.
At present, beam shields are only available for SF2 mecha. The shield generator itself weighs only half a ton, making them lighter than physical shields for the same damage blocked.
Whereas physical melee mecha weapons can damage physical shields, beam shields damage the weapons instead. Beam shields also regenerate after every battle, whereas physical shields have to be repaired every time they are hit.
These properties make beam shields often a superior choice to physical ones. Note that, on the other hand, you cannot install any equipment in beam shields; for example one cannot install a weapon in a beam shield itself to avoid the to-hit penalty of mounting it on the shield arm, whereas physical shields provide one CMX of space.