There's a mythology in 2/1 that you have to play 1NT forcing, or at least, in the 21st century version of the myth, semi-forcing. In principle, you can miss game or have a wide variety of other auction misses if you don't, but like everything else there's a trade-off, and there are a lot of very good players who play it non-forcing. (FWIW, I am now in the non-forcing camp, though I will tend to treat it as semi-forcing if the rebid that entails is palatable.)

If you play it forcing, then you have to find a rebid when you open 1♠ and have something like 5♠ 3♥ 3♦ 2♣ shape. Here, you'd have to rebid either a 3 card diamond suit or a 2 card club suit, with the 3 card diamond suit being the standard "lie." This has implications, too (think "pink ink" in The Cat in the Hat Comes Back, if you read that as a child, or want to Google it for fun), but let's not worry about those for now. It's even worse if you have 4♠ 5♥ 2♦ 2♣ shape and it goes 1H-1NT, because you really have to bid a 2 card club suit, and it's going to take some Voom to fix that.

If you play it semi-forcing, then you can pass with a balanced minimum as long as you don't have something in the good 13-14 HCP range (assuming 12 HCP to open if balanced and a 15-17 HCP 1NT range). You only go through the gymnastics of manufacturing a minor suit rebid if you have one of those goodish hands and don't want to miss game opposite 11 HCP balanced or an invitational range hand with 3 card trump suit (most 2/1 players begin with 1NT when holding a 3 card limit raise hand).

If you play it non-forcing, you're allowed more judgment. With 5♠ 3♥ 3♦ 2♣ shape, you pass some goodish hands rather than bidding 2D on 3 small diamonds. If you have the diamond KQx and a goodish hand, you bid the diamonds.

Two other notes:

1) As I suggest above, potentially manufactured rebids cause additional problems, and the pink ink next spreads to responder's rebid. Without knowing whether opener's rebid suit is real, responder will sometimes not know whether to pass it or to take opener back to their major with only 2 card support. If responder does so while actually holding 1 more card in opener's 2nd suit than opener's first, this is called a "false preference." For example, after 1♠ -1NT-2♣ , responder might now bid 2♠  with 2 spades despite holding 3 clubs, and it might even be right to do it with 4 clubs. And this is not even the worst of it; imagine if 2♣  might only be a 2-card suit. Some experts choose to bring out the Voom and others tell the Cat "no thanks" and let their minor suit rebids mean that they actually have the suit. 

2) In a formal club game or a tournament game, if you play that 1NT is semi-forcing or forcing, this must be announced at the time of the bid by the partner of the 1NT bidder. (Along with the opening 1NT range and transfer bids, these are the three announceable calls that are part of most Standard American or 2/1 systems.)  Because of the way the ACBL defines semi-forcing, 1NT must be announced as semi-forcing if it can have 11 HCP, even if the partnership has agreed to treat it as non-forcing.