Redirection increases the Maximum Shield Capacity of a Warframe. It is the first normal Mod that all new players receive in Warframe, where it is automatically given upon activating the Orbiter's Mods console.
The increase to shield capacity provided by Redirection is applied to the shield capacity of the Warframe as if it were Unranked. Then the bonus shield capacity provided by the Warframe's current Rank is added to the total shields.
For example: If a maxed Redirection is applied to a level 30 Frost Prime, the Unranked shield capacity (175) is multiplied by 5.4, giving 945. Then the 350 bonus shields provided by Frost Prime being Rank 30 are added, for a grand total of 1295 shield capacity.