Yes, it's pretty much the same difference as "can" and "could" in English.
In this specific case both words do a very similar thing, in that they both turn a direct request into a more polite question.
"Könnten" is the subjunctive (in German: "Konjunktiv") of "können". One of the usages of Konjunktiv is expressing courtesy. As explained in this article "könnten" is the even more polite, humble form.
So these three sentences have technically the same goal:
"Sagen Sie mir bitte wie spät es ist!"
is very direct, almost rude. It's a direct requests and could be considered an order, if it weren't for the word "bitte".
"Können Sie mir bitte sagen wie spät es ist?"
is already more polite and softens the "demand" by asking if they could do this for me.
"Könnten Sie mir sagen wie viel Uhr es ist, bitte?"
is the most polite form of the three. It's effectively saying "I'm not entitled to this action, but I'm still asking you to do it, if you would be willing to".
In another context, they choice between the two words could change the actual meaning as opposed to just bein a more polite form of the same, however:
- "Wir können jetzt gehen."
- "Wir könnten jetzt gehen."
The first one indicates that we are ready to go now. The second one indicates that we could go now, but there's something that's stopping us (even if it's just the decision to actually go).