Both mean yield — but only one requires you to actually stop.
Yield (upside-down triangle): you yield. You may roll forward slowly and enter the junction if it's clear. No stop required. Stop (red octagon with STOP): you must stop completely, wheels not moving, at the stop line even if the road is clear. Then you look both ways and go when safe.
Police checks usually focus on whether the car comes to a complete stop at the stop sign. A 'rolling stop', where the car never fully stops, counts as an offence, fixed fine of SEK 1,200.
Yield: you approach a T-junction, look left and right while braking, no traffic. You roll out without stopping. Stop: same junction, stop sign. You stop first, then look, then go. Both are legal in their own context but wrong in the other.
Stop signs are placed where visibility is severely limited or where traffic flow makes it hard to judge the situation safely. Not stopping there is more dangerous than at an ordinary sign, the location itself requires a full stop.