They show a two-suited hand where the suit opened is at least (and usually) five cards and the second suit is a minor and might be only four cards depending on vulnerability, opening position and perhaps other considerations. Strength is about 5-10 HCP and not good enough to open at the 1-level.
What I describe here is an extension of Muiderberg that allows opening with both majors. To untangle this when partner happens to have a strong hand, the rebids after a 2NT response are changed to the following:
3♣/♦ Minimum, natural 3♥ Minimum, both majors 3♠ Maximum, both majors, 5-4 pattern 3NT Maximum with either minor 4♣/♦ Maximum, natural, 6-card minor 4♥ Maximum, both majors, 5-5 or betterAnother wrinkle is that after a 2♥ opening, the 2♠ response is now pass-or-correct and no longer promises a long spade suit.
As a general rule you don't want to correct to another suit unless holding a singleton or void in the opening major. Let the opponents guess what to do.
Finally there's the question of what to do when the rebid is 3NT and you don't want to stop there. If you happen to play Optional Minorwood, then you may bid 4♣ or 4♦ as a key card ask for that suit; here responder's first step means "that's not my minor", and asker's bid of the subsequent step means "then please answer for the other minor".
If you don't like Optional Minorwood then use 4♦ to ask: 4♥ = clubs, 4♠ = diamonds. Bidding 4♣ instead would be invitational and nonforcing with tolerance for both minors.