Thumb Impression in Extortion

It is a legal principle that the burden of proving the fact alleged is on the person making the allegation. So, if he can prove that signature was taken by force, then it amounts to extortion. Otherwise, court presumes that the document was duly signed. 

Any thum impression, especially disposing a property has to be affixed before a Notary, duly recording the fact that the person sining (affixing the thumb impression  was explained about the document that he/she executed. ALso the executant should be identified by an Advocate and both Notary and Advocate's seal and signature have to be available.

 CAN U TELL ME WHAT IS A "NOTARY"?I AM NOT CLEAR OF THIS..