A stranger who does something significant in a dream — hands you something, speaks directly to you, or guides you somewhere — is read as an unknown factor entering your life from outside your usual circle, rather than through anyone you already know.
A kind or helpful stranger is read as unexpected help arriving from an unfamiliar source — an opportunity, piece of advice, or connection you wouldn't have predicted coming from where it does. A threatening or unsettling stranger is read as an unidentified risk — something you sense but haven't been able to name or locate yet in waking life.
A stranger who feels oddly familiar, as though you should know them but can't place them, taps into a fairly standard idea in general dream psychology: unfamiliar figures in dreams sometimes represent an aspect of yourself you haven't fully acknowledged, rather than an actual outside person at all.
A stranger who simply appears in the background without interacting is read fairly neutrally — background awareness that your circumstances involve more moving parts, and more people, than you're currently tracking closely.