Gibson, yes, surely in the case of a teenager it's usually the parents. But not everyone who fills out the form is a teenager, so other people might have another friend or relative who is willing to accept financial responsibility and show proof of ability to pay. It's typically a significant legal and financial commitment per year, depending on tuition and fees at the individual school, and can be enforced in court. (That is, recruiting international students who pay full cost, not a reduced rate, is a significant source of income for many colleges.)
If you need more help, I would suggest just doing a web search (DuckDuckGo is safer than G**gle) with the number of the form in quotes (if it has a number), plus terms like 'student visa,' 'financial responsibility,' and 'sponsor' or 'sponsorship.' The form, and its number, may be different for a student visa compared to some other longer-term visa.
Or it may be just a form created by the university, in which case the university should be able to help if you need help. I imagine every admissions office will have people who are used to answering questions from international students, and even if they're not in the office this month, they may be checking e-mail occasionally.
Is it kind of similar to this, as opposed to a government-issued form with an official number?https://www.uhd.edu/admissions/documents/intl...