What does “ударение” mean in Russian pronunciation?

In Russian, an О is only pronounced as a full О when it carries the ударение — the stress. Move the stress away and the same letter drifts toward an “ah” sound. That is why докуме́нт sounds more like дакумент: the stress sits on the last syllable, so the unstressed О reduces. As a rule of thumb: no stress, no full О — and the spelling-to-sound gap closes across thousands of everyday words.

The rule splits neatly in two: О under stress stays О — о́фис, ко́фе, телефо́н keep their full О sound. О without stress drifts toward А — докуме́нт, рестора́н, паспо́рт all shift. In the clip the teacher spells “д, о, к, у, м, е, н, т” letter by letter, then pauses: “Это О или А?” — is it an O or an A? The answer is both: written О, spoken А, because the stress is elsewhere. The same rule explains why Москва sounds like Масква and хорошо like харашо.