If a thing is indubitable, you cannot honestly doubt it. It is not merely likely; the speaker is saying doubt has no room left. It is a formal, slightly old-fashioned word — you find it in careful writing, speeches, and people who want to sound very sure. In ordinary talk, people say no doubt, obvious, or clear instead. Though actually, calling something indubitable is a strong move: you are not just saying you believe it, you are saying a reasonable person should not argue. Use it when the fact is solid; used loosely, it can sound like you are showing off.