When a subject is restrained, they have a movement limited by whatever is restraining them. (typically 0-5ft) and cannot attack. Some causes of restraint will impose additional limitations. Unless specified otherwise, the subject can still perform some minor actions such as taking something out of their pocket, grabbing at something, nocking something over, etc. Actions while restrained can sometimes require a DEX check (or a STR check if it is more appropriate) for actions which otherwise would not.
Attacks against the subject have advantage.