that

What does 'that' mean here?

Given the statement of the first law above, we can conclude that an object that is
accelerating must be experiencing a force. In turn, from the first law, we can define
force as that which causes a change in motion of an object.

Imagine playing catch with either a basketball or a bowling ball. Which ball is more
likely to keep moving when you try to catch it? Which ball requires more effort to
throw it? The bowling ball requires more effort. In the language of physics, we say
that the bowling ball is more resistant to changes in its velocity than the basketball.
How can we quantify this concept?
Mass is that property of an object that specifies how much resistance an object
exhibits to changes in its velocity,