A guy from Prague gets on a train and enters a compartment where there is a guy from Ostrava sitting and says to him: "Posuň se až k voknu." The guy from Ostrava does nothing, only looks around baffled. The Praguer repeats: "Posuň se až k voknu." to which the guy from Ostrava replies: Tak už kvokni, ty ****.
(If you have not understood, it's a joke and the sentence "Posuň se až kvoknu." means "Move yourself after I cluck. [where "cluck" is the sound made by a hen]). :-)
This is one of the sound changes that happened to Czech in most of the country around the 16th century. Together witch changes like
ú -> ou
or ý -> ej
Some of the changes were later accepted into the standard Czech during the national revival when Czech returned to be used by educated people and in fficial documents.
For example, people in Moravia may still say lúka or múka but in Bohemia we say louka and mouka. At the beginning of the word, however, it was not accepted into the standard and later mostly returned back
úřad -> ouřad -> úřad
ústa -> ousta -> ústa
This ú->ou sound change (except at the beginning) was accepted into the standard Czech, but dialects of Moravia may differ.
být -> bejt
mýt -> mejt
okno -> vokno
on -> von
were not accepted into the standard but people in Bohemia continued to talk that way.
Note that many similar sound changes happened to English around the same time. The main difference is that the English orthography does not really reflect exact sounds so the chsnges are not visible but are responsible for some of the mess in modern English orthography. See, e.g., Great Vowel Shift. Shakespeare used to sound very differently in the 16th century.
According to new research the "prothetic v" feature may be disappearing among the young generations under the influence of standard Czech at school and in media https://ct24.ceskatelevize.cz/veda/2540368-vopice-uz-vokno-nevobehne-z-cestiny-podle-jazykovedcu-mizi-proteticke-v
The most important point is, this lesson is about how people speak in colloquial situations. Not about what is "correct".
It also does not represent the dialects of Moravia or Silesia. None of the dialects in Bohemia, Moravia or Silesia are "correct". All have their specifics in morphology and vocabulary. In the latter groups one can identify transitional features to Slovak and to a lesser degree to Polish. Again, it does not make any of the dialects "more correct" than the others.
For those, who keep telling us that it can NEVER be written...
These are from the literature (e.g. the translation of Steinbeck's The Grapes of Wrath) and from newspapers. People DO write this when showing how people really speak. Obviously, in the case of the translation, it is used to mimic the vernacular dialects of English where the original author used one.