An omer refers to an ancient Hebrew measure of grain (roughly 3.6 litres). Biblical law forbade any use of the new barley crop until an omer was brought as an offering to the Temple in Jerusalem. The Torah also commanded: "And from the day on which you bring the offering…you shall count off seven weeks. They must be complete." This commandment led to the practice of the Sefirat Ha'omer, or the 49 days of the "Counting of the Omer," from the second day of Passover to Shavuot.
Lag B'Omer is the shorthand way of saying the 33rd day of the omer. It is celebrated to commemorate the day that a plague ended in which thousands of students of Rabbi Akiba, a Talmudic scholar, died during the Counting of the Omer. The period of counting is traditionally observed as a period of mourning. The mourning, however, is set aside on Lag B'Omer, making it a day of special joy and festivity.
Since Lag B'Omer is not mentioned in the Torah and only hinted at in the Talmud, there is no formal ritual but rather a series of customs that people find attractive and meaningful. Here at Temple Beth Am, we, like many Jews, celebrate through festive gatherings and bonfires.