Rustのusers forumでも数学でのboundsのようなものと説明があるね
つまり制約ではなくて境界や限界それらによる範囲を表しているんだよ
制約(constraint)とは概念が全く異なる用語であるためこの違いは重要かと

Q. What is “trait bounds” exact meaning?
Especially I can't understand the "bound" part.
Does it mean "bind"? Or does it mean "the border for the distinct from others"?

A. Yes.
In mathematics you might hear about “upper and lower bounds” for a numeric variable — it's the same sort of thing,
setting boundaries for what values (types) the type-variable may take on.