[ehhh]

Please, help me understand this simple auction.
After a 2♣ opener, what should 2NT by partner mean?

[awm]

Assuming 2♣ strong, it seems like overcalling 2NT would be a pretty unusual call.

So I'd take it as showing the minors.

If you mean a 2NT response to partner's strong 2♣ opening, some people play this as a positive with hearts and use 2♥ as an immediate negative. Another alternative is to have 2NT showing a good hand with a lot of slow cards (i.e queens and jacks) that might be better off declaring a NT contract. At one point I know a pair using 2NT response to show "8+ points but no aces or kings."

[ehhh]

Sorry, I should have been more exact.
The 2 ♣'s opener by the opposition
did show a strong hand (19+hcpts)
and the 2NT was an overcall by partner.

[blackshoe]

Absent discussion, both minors, probably weak (but with decent playing strength).

[neilkaz]

I would certainly think this is a normal unusual 2NT showing the lower two unbid suits...ie ♣ & ♦ since no real suit has been bid.

[the hog]

As has been stated, UNT showing both minors. Perhaps he might have intended it as some M/m 2 suiter.

[barmar]

I don't know if there's a standard treatment (other than the obvious "minors"), but I think it makes more sense for it to show a more general two-suiter. Over 1-level openings, there are a variety of ways to show various different two-suiters (Michaels and Unusual 2NT over suits, DONT, Cappaletti, Astro, Hello, etc. over 1NT, Truscott, Mathe, CRASH, etc. over strong 1♣), but few partnerships have such detailed systems over 2♣. If you only use 2NT when you have minors, it won't come up very much (1 out of 6 two-suiters) and you're still left with no other way to show the rest of them. So perhaps it should be any two-suiter.

One thing, though. If you're going to bid a two-suiter over 2♣, I think it should probably be at least 6-5, not just 5-5 (and certainly not 5-4, as you might be with some of the conventions mentioned above). You'd like to have a decent chance of finding an 8-card fit to come in over a powerhouse opener. Advancer responds by bidding his cheapest 3+-card suit, and overcaller passes or corrects. The screw case is when overcaller is 1=5=1=6 while advancer is 4=1=6=2 -- they end up in a 5-1 fit rather than their 6-2 or 6-1 fit.

[SoTired]

there are numerous 2-suited conventions used against a 2C (or Precision 1C) opening