Not every application involves an interview, and where one exists its purpose depends heavily on the system.
The United Kingdom
Interviews at some universities are academic and subject specific. A problem is presented and the candidate works through it in front of the interviewer.
The point is not to arrive at the right answer. It is to show how you think: what you notice, what you try, how you respond when told your approach is wrong.
Candidates who present rehearsed answers do poorly, because the format is designed to move past them within a minute.
The United States
Interviews are frequently optional and often conducted by alumni rather than admissions staff. The conversation is about interests, motivation and fit rather than subject knowledge.
The weight varies enormously. At some institutions the interview is a meaningful part of the file; at others it is a courtesy. Institutions publish this.
What actually helps
For an academic interview, practise thinking aloud on unfamiliar problems. Most students have never narrated their reasoning to another person and find it far harder than solving quietly.
For a conversational interview, be able to talk specifically about the things already in your application. Vague enthusiasm about a subject reads worse than one concrete thing you did with it.
The most common mistake
Answering the question you prepared instead of the one that was asked. Interviewers notice immediately, and it is more damaging than not knowing something.
Saying you do not know
In an academic interview this is acceptable and often expected, provided it is followed by an attempt. Silence is the problem, not uncertainty.