what is exactly mean by antigen. is it  foreign microbes that enter ur body????

An antigen is a substance or a molecule which, when introduced into the body, triggers the production of an antibody by the immune system. Yes, most of the times a substance that acts as an antigen is foreign that enter our body from external environment such as microbes, but sometimes internal substances also act as antigen such as tissue toxins. The immune system will try to destroy or neutralize them.