Hello Mukesh
First learn what is greatest integer function here https://www.teachoo.com/7123/2217/Greatest-Integer-Function/category/Different-Functions-and-their-graphs/
Then, check the intro again. https://www.teachoo.com/3765/697/Ex-5.1--19---Show-that-g(x)-=-x----x--is-discontinuous-at-all/category/Ex-5.1/
Let me know if there is still any doubts.