What should i understand by css in practical way??

Answer 1

in other words

you prepare template for your html files so by using css every colors font size and style will be same in all html files

Answer 2

html is notepadd
css is wordpad

by the way everything you do in css you can do it html but you have to write more tons of codes

instead of writing styling codes in all html pages you write it in css file and call that file from all html pages to gain tons of time for coding