I would think it means that a person is very determined to accomplish something. They might not be very strong physically, but they still get things done. Or, I am not sure of the context here, so it could be referring to a person's last will and testament. Maybe it means that the will's instructions should be followed alone, without listening to other opinions.

Usually when I see it, it describes some act that would normally not be physically possible for the person doing the act. However, the person was able to do it anyway using the force of their own willpower. This is sometimes referred to as "mind over matter". You accomplish something because you believe strongly enough that you can, despite obvious difficulties.