Want to know what is it used for

It generally refers to replacing female hormones as women reach menopause, when some hormones are depleted. It is only done under medical supervision, and is controversial because of possible serious consequences.
In rare cases, it can be a treatment for anyone who is missing hormones naturally.
Why do you ask this under weight loss?

Yes, I want to know, is hormone replacement therapy can help me to lose weight.

No - and even if it did, it's not worth the possible dangers, such as breast and endometrial cancers. The controversies seem to stem from differences in the amount and forms of hormones used. and the delivery systems. And studies of weight gain and loss showed no difference in women who had HRT and those who didn't.

The ONLY known ways to lose weight:
Eat less, and cut out sugar entirely
Exercise
Eat healthy meals that combine food groups in one meal (don't eat a piece of chicken one meal, and some carrots 4 hours later. Cook food and put it into small containers to make small servings, but combined)
Drink plenty of water or weak drinks (although the old '8 glasses a day' is now discredited in studies)

No, I would not even know, who would suggest such a thing, surely not any competent doctor