It is very similat to English "a" and "an". In speach, you can use them interchangeably. For example - V hotelu (at the hotel, literaly inside the hotel) uses v because for a czech person it is very easy to fluently say vh sound. Sou the two words sound like vhotelu rather than v hotelu. Whereas in "ve meste" in the city, saying vmeste sound would be harder for us. So feel free to use which ever you feel comfortable when speaking, writting might require a bit of drilling

link to Czech handbook: http://prirucka.ujc.cas.cz/?id=770