Resistance refers to the concept of struggle in the martial arts, which has both positive and negative connotations. It may be looked at as a negative aspect to encounter resistance from an opponent when trying to apply a technique, because that resistance may cause you to fail. It may be looked at as a positive aspect such as in sparring, where resistance is the idea that my opponent is inherently trying to cause my techniques to fail, so I must be practiced at adapting and navigating around their resistance to apply techniques.

This is heavily related to the concept of "Ju" in Judo, which quite simply is "non-resistance". That if an opponent can put up resistance to your actions, you must be skilled in neutralizing that resistance. If you can neutralize any resistance your opponent puts up against your technique, defeating them regardless, then you have made your opponent "non-resistant". It's not that your opponent isn't resisting, it's just that due to the skill of "Ju", their resistance no longer matters.

For example, if your opponent pushes you and you push back, this creates resistance. If when your opponent pushes you, but you pull them while they are pushing, causing them to trip and fall, you have applied the principle of "Ju". Rather than fight their resistance, you neutralized their resistance.