A Redirect is an action that occurs during the night phase in the form of "Redirect X to Y" where, without any intervention, will change X's target from whomever it was to Y. The power of the redirect varies greatly depending on its placement in the order of actions, especially in relation to the block.
Additionally, the way a spy is redirected varies from host to host. For example, if A spies B and X redirects A to Y, some hosts will tell A that B is Y's role. Other hosts will tell the spy that he was redirected and that Y is Y's role. Always confirm with the host on which method is used.
Some actions are usually unable to be redirected. For example - RID actions, redirects, resurrects, many ODTG actions, ask host a yes/no question, role inherit, day actions such as vote manips/redirects and lynch saves, and unconventional, innovative roles. Redirects may also fail if targeted to a player who did not act due to being blocked/trapped, not having a night action or not entering an action (e.g - a goodie kill who abstained on N1.)
Some hosts will inform the redirect role when their action fails. This allows the redirect role to narrow down the possible roles of their target, and thus use their role as a weak spy.