Hi i jave red patch on my tongue. i don't know what is that?

Geographic tongue: Red patches can develop when the normal filiform papilla covering the tongue change. They can come back and everything will look normal, or the patches can move around the tongue and this is called geographic tongue. In any case we we don't know what causes it and it is usually nothing to be concerned about. For a proper diagnosis see your dentist.

Hard to say: Red patches on the tongue can be anything from a healing burn or scrape, to normal tissue variation, to a benign condition called 'migratory glossitis', to any other number of things. If it is new, and does not go away after 2-3 weeks, have it evaluated by your dentist. Without more information, it is impossible to accurately diagnose.