how important is icse when you want to apply undergraduate for harvard stanford ucla oxford and icse for your life

I think you mean to say IB board/ IGCSE

There are primary 2 way to get an admission in a US university in undergraduate.
1) whatever board you have and you give SAT, obtain a good score and you have a good portfolio of your high school work you get an admission

2) You get admission in IGCSE/IB Board and give A levels examination (equvalent to 12^th in our country) and then apply