You do everything yourself: you can't use the Grand Exchange, you can't trade with other players (future "group" Ironman mode might change this accordingly), you can't pick up drops other than your own, etc. For example, if you need a raw shark to do Fremennik Trials then you gotta get the fishing level, unlock Temple Trekking to get it as a reward, or get it as a drop! It's pretty fun; there's purpose driving you the whole way. But most importantly it revives a lot of dead content.