What are Bound expression and Unbound expression in lightning salesforce?

Bound Expression: It is represented as {!v.str}. Whenever the value of the string is changed, this expression will reflect the change and also affect the components where it is used, we can say the value change dynamically through this expression.
Unbound Expression: It is represented as {#v.str}. Whenever the value of the string is changed, this expression will not reflect the change, we can say the value remains static through this expression.