As described in the Basic Information section the weak typically refers to the top half of the blade including the point. Sometimes translated as "foible", the weak of the blade is fast moving but has little mechanical advantage.
This is neither a positive or negative trait, it is simply the nature of the way swords work. The tip of the sword moves very quickly when we swing the sword, making it the best part to cut. It is also quick to move around; a small movement of the hilt results in a large motion of the point, so we can use the point deceptively.
On the other side of the coin, it is difficult to exert significant pressure in the bind with the weak of the blade, and it is easily set aside.
When using the weak we should consider these traits to make the most of our techniques. Typically, for example, we avoid parrying with the weak, instead using the Strong.
Related to weak is the concept of being Soft in the bind.