Won’t it be commercial use if I will use github pages as portfolio to apply? Or prohibited commercial purposes in terms mean e.g. e-commerce web-site?
Using it for your portfolio is perfectly fine and not a violation of the terms of service. You can find a summary of the prohibited use cases on the About GitHub Pages help article:
GitHub Pages is not intended for or allowed to be used as a free web hosting service to run your online business, e-commerce site, or any other website that is primarily directed at either facilitating commercial transactions or providing commercial software as a service (SaaS).
Additionally, GitHub Pages sites must refrain from:
- Content or activity that is illegal or otherwise prohibited by our Terms of Service or Community Guidelines
- Violent or threatening content or activity
- Excessive automated bulk activity (for example, spamming)
- Activity that compromises GitHub users or GitHub services
- Get-rich-quick schemes
- Sexually obscene content
- Content that misrepresents your identity or site purpose
(Copied from https://docs.github.com/en/github/working-with-github-pages/about-github-pages on 2020-08-24 14:49 UTC)