OK. Now what's an omer?
It is a unit of measure -- like a pound or a kilo. That is how sheaves of barley were measured. The Israelites were commanded to bring an omer of barley to the Temple on the second day of Passover. If you don't believe me, read this week's Torah portion, Emor. It says it right there. Then they were supposed to count 49 days from that day, until Shavuot. Lag B'Omer falls on the 33rd day of the counting of the omer. Get it?