A transfer in bridge is made after a notrump opening by bidding the suit below your actual suit. If you have a 5-card heart suit, you bid 2♦. If you have 5 spades, you bid 2♥. The bid is a command to partner to bid the suit above the one you bid. Your second bid, or pass, shows the strength of your hand.
Weakness takeout using transfers
You don't have enough for game, and your suit is pretty horrible, but your hand is much more suitable for spades than notrumps. Transfer to 2!s and pass. The great thing about transfers with a weak hand is that partner, who is stronger, often becomes declarer.
Choosing the best game
You have enough for game with this hand so tell partner. The beauty of this convention is that you can now bid 3NT. First tell your partner that you have 5 spades by bidding 2!h, then tell partner that you have enough for game. It is up to partner to choose the final contract. - 3NT or 4♠.
Weak Minor Hands
If you have a very weak hand and a 6 or 7-card minor you should bid 2♠. This does not show spades as you would have bid 2♥. 2♠ demands that partner bid 3♣. You now bid your minor suit and partner must pass.
South shows a weak hand with long diamonds.
Don't transfer with a 5-card minor. You can call that a balanced hand. You need a 6-card suit at least.