As described by Tomura Shigaraki, Shock Absorption gives the user the ability to reduce the effectiveness of physical blows by absorbing the shocks produced by them. This power was specifically given to the Nomu to combat All Might's incredible physical strength. While it hasn't been fully demonstrated, it has been noted that Shock Absorption can also absorb attacks and fire them back like a recoil.
This Quirk does not give the user the ability to completely nullify blows, meaning there is a limit to how much punishment the user can withstand.
Tomura even stated that the primary method to overcome this power is to slowly lacerate them.
The U.S.J. Nomu was created to destroy All Might by absorbing his enhanced blows via Shock Absorption. This Quirk allows the Nomu to negate most physical blows, including fully powered One For All smashes.