In policy debate, defense refers to argument which has no implication other than mitigating an argument made by the opposing team.
For example, "A will not cause B" is a defensive argument, while "A will cause B" is an offensive argument.
Defensive arguments may often be conceded to "kick out" of positions. For example, a team wishing to kick out of their politics disadvantage could concede the argument "Fiat takes out the link." Although this argument is frowned upon by the community, the fact that it is conceded gives it 100% percent weight in the round and thus takes out any offence the other team may have on that flow. For example, an impact turn on a disad with no internal link becomes irrelevant.
Why most coaches and judges encourage you to have more defense is because the offense is exactly that -- offense. So if the negative goes for only one argument in the 2NR, and argues that there is no affirmative offense on the flow, they essentially win the round, because at any risk of the CP, D/A, K, whatever, solving, they win the round.