Poker defense is just as important as aggressive play in no-limit hold'em. It's used not only to defend our small blind (SB) and big blind (BB), but also our game after the flop.
If you have the SB, you can call the BB with almost any playable cards. However, it is important to keep in mind that the BB player could have above-average cards and raise enough to discourage most players to call. In this case, it is important to consider the cards you have and how much you are willing to invest in the game. If you were calling or raising with cards which were not very strong, it's better to simply fold. However, an important factor to consider is also how many players called or slightly raised the bet. If we are the only ones who called the BB, it is possible that the other player raised the BB only to intimidate us and make us fold; it's up to us to correctly identify the opponent's intentions. This depends on how well we have read him or her. We could even raise his or her raise if we are sure about ourselves; this could then intimidate the BB player and make him or her fold. If we have average or subpar cards and several players joined the game before the flop and even raised the bet, then we should leave our SB and simply fold. In that case it's reasonable to assume that at least one opponent really has good cards. And that would mean that we would just be throwing away our chips for nothing. It is very important to be able to make the right decisions at the right time in poker. This is based on poker math and an assessment of the probability of our success, but sometimes even more on our intuition and ability to correctly read our opponents. It is important to be patient and avoid reckless or vain actions.
Consider the following example. We have 7-9 in hand, called the game and after the flop we see 2-7-J on the table. If a player who already raised before the flop raises again, then it's clear that he or she would beat us even with just a higher pair, meaning a J, two 2s, or a 7 with a higher card than we have. However, the reality could be completely different. He or she might have none of the above combinations; they could just be bluffing, or waiting for a pair to go with a high card they have in hand. It is easy to throw away your cards after the flop, but an aggressive player could very easily identify our strategy and we would end up quickly losing all our chips. That is why it is important to often defend our combination in the same way as our blinds. That can be done not just by calling, but also by starting a "counterattack" and raising our opponent's raise. If we are defending our blinds and have the feeling that someone is just stealing them from us by raising, we can also raise. If they call, we can raise even after the flop; in that case, if the player was truly just stealing then they'll fold. Keep in mind that, just like us, our opponents might also be using various poker tactics and tricks. They could see through our move and, believing that we are merely defending our blinds with no winning combination, raise our raise. That is why it's important to defend our blinds only when we have suitable cards – at least one should be above 10, or they should be connectors, or of course if we have a pair. Defensive play is very important, but when defending we also need to keep our options in mind, i.e., how well have we read our opponents, how many chips we have left and what are our cards. Furthermore, when doing all of the above it is important to maximally control our body language and avoid making stubborn or vain moves.