For anyone who has learned martial arts, you know that in order to make your moves effective (e.g. strikes, blocks, punches etc), you need to practice your basics to certain standards.
For example, a horse stance is one of the fundamental stances I’ve learned in my karate classes. When you do a horse stance, your feet are adjusted to a position that’s slightly more than shoulder width, and your knees bend. Is it easy? No, your knees and thighs start shaking after you correctly hold this stance for some time.
If you don’t pay enough attention to making it right, or simply and unconsciously want to avoid the leg tiredness, you’ll easily lose your horse stance, instead, just stand with your knees slightly bent. If you practice your horse stance like this often enough, it becomes part of your muscle memory.
Why is it important to hold the gestures and moves up to certain standards in practice? In our karate classes, everyone is very nice and accommodates each other’s needs — they go slow as needed, or just show a punch instead of punching into your nose for real. You have enough time to think and react. However, things change when you are in a self-defense situation, there’s not much time for you to think, plan moves, let alone have a friendly reminder from an instructor on where to strike. If you decide to defend yourself, you rely on knowledge you can immediately remember and what’s rooted in your muscle memory. You may not even do half as well as the way you perform in a classroom.
Therefore, set your bars high enough for your practices and let what you’ve learned and practiced become a part of you. I think that’s what people mean by “Perfect practice makes perfect”.