Huh?
Doing nothin requires absolutely no energy...but, instead of idling,its ofcourse better for you to benefit the day by contributing something good to the community,
which is why your mother-in-law said that to you.
Earn some money is ofcourse better than earning nothing,right? So, thats just some way of your mother-in-law to advice you so that you wont be such a lazy person.