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♦ then asks: 4♥ = clubs, 4♠ = diamonds) 4♣/♦ Maximum, natural, 6-card minor 4♥ Maximum, both majors, 5-5 or betterWhen the rebid is 3NT and you happen to play Optional Minorwood, then you may instead 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".
Another 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.