We count the seven weeks between Pesach and Shavuot as days of preparation from the exodus from Egypt to receiving the Torah on Shavuot. Every night of the Omer, we bless “Who has commanded us to count the Omer” and then announce the appropriate day of the Omer and the total number of weeks that have gone by.
But what if you forget to count one night? The authorities are split. Some say that counting the Omer is a package deal. If you miss one day, you cannot continue counting with a blessing because you cannot make up that missing day. You can still keep count, but you cannot fulfil the entire mitzvah and so the blessing no longer applies.
Others (the Tosafot, the Ritva) say that each night of the Omer is its own mitzvah. So if you miss out one day, you can still count with a blessing on subsequent nights.In practice, if we miss a day, we do not make a blessing on the Omer for the rest of the season.