The oldest, most common transition there is. Where a hard cut implies "time is continuous," a dissolve implicitly says "time has passed" or "these scenes are connected" — which is why it shows up so often in flashbacks and montages.
In Premiere Pro it's as simple as overlapping two clips on the timeline and dropping in a Cross Dissolve transition. Underneath, it's just clip A's opacity going 100→0 while clip B's goes 0→100 at the same time.
The longer the dissolve, the stronger the "time has passed" feeling gets. A quick 0.5-second dissolve and a slow 3-second one are the same technique but read as completely different statements.
When to use
Don't dissolve between unrelated scenes — viewers unconsciously search for a connection and end up confused when there isn't one.