(An omer is a measure of grain, like a bushel.) Each night during the 49 days of this seven week period, the count of the days is recited with a blessing--this ritual is called "Sefirat HaOmer." In Kabbalah (Jewish mysticism), each of the 49 days also corresponds with the purification of a specific quality within ourselves that connects to the divine.
There is also a modern custom, based on the Sefirot and counting the Omer, to similarly count the days from Tisha B'Av to Rosh Hashanah.
These qualities are called "Sefirot," and remembering which qualities correspond to each day has been part of the ritual of counting the Omer since the Middle Ages.
This app will help you count the Omer from Passover to Shavuot by keeping track of the count and giving you the blessing and the Hebrew for each day's count.
What is the Omer? Exactly seven weeks fall between Passover and Shavuot, marking the transition from the redemption in Egypt to the revelation at Sinai.