One of the mitzvos mentioned in our parashah is the mitzvah of Machatzis Hashekel. The Chinuch (105) tells us the about the mitzvah:
Each year, on the first day of Adar, Bais Din would begin reminding all men over 20 to give a half-shekel to the Kohanim.
The collected money was used throughout the year in the Bais Hamikdosh for various purposes, including Karbonos, firewood, the Red Heifer, and the Omer.
The Chinuch explains one of the reasons behind the mitzvah: Hashem wanted each individual to have an equal share in the sacrifices offered throughout the year. By each person giving the same half-shekel amount, the poor are now on par with the rich.
The Rabbis (Megillah 29a) instituted that the Shabbos preceding Rosh Chodesh Adar, the Parsha of the Shekalim, be read to remind the people of their upcoming obligation.
Nowadays, even though we no longer have a Beis Hamikdosh and the original mitzvah of Machatzis Hashekel does not apply, we still read the Parsha and give the half-Shekel each year.