"hats off to u" means that you are expressing your admiration, praise, or respect to the person. It evolved from the action of actually taking off one's hat as a sign of respect for a person. It means Kise ki prashanasa ka Abhar vyakt karana (प्रशंसा का आभार व्यक्त करना)

Removing your hat is a British custom and indicates respect. We remove our hats when the teacher enters, when the Queen arrives, when any important person shows up. It is also an expression of congratulations. When someone has achieved something significant you remove your hat out of admiration. Sometimes whole groups will take their hats off to congratulate an individual. Such is the origin of the expression.

Yes, it's a phrase used to express admiration or respect at what someone has done. "Hats" refers to "our hats" or "everyone's hats". For a single speaker, use "I take my hat off to you", "My hat's off to you" or "I tip my hat to you." You can also use "doff" (= remove, literally "do off"), which sounds rather quaint: "I doff my hat to you."

Arwa..it sounds too crazy that muslim girls can't take off there hijab but in there are crores of muslim and never saw them wearinf it or it is compulsary for them.

Muslim women who are convinced of wearing hijab never take it off. I'll never ever take it off. :)