In the time of the Bet haMiqdash, the Temple of Jerusalem, daily communal sacrifices (qorbanot) were offered on behalf of all the Jewish people. This national representation was not just theoretical or depending only in the right intention of those in charge of performing the sacrifices, i.e., the Kohanim. It was also conveyed in a very practical way: Every year in the month of Adar a special tax was collected: machatzit hasheqel or "half a sheqel". The half sheqel was a flat tax, same amount for poor and rich. And since everyone gave the same amount, the collection of the sheqalim (plural of sheqel) also served as a annual demographic cense. The money collected in this communal fund was used to purchase the animals for the qorbanot, and in this way every one had a equal share in the offering of the sacrifices. The daily communal sacrifices and the sacrifices of the different musafim(Shabbat, Rosh Chodesh, holidays) were purchased from the money of the sheqalim. The half sheqel was collected during the month of Adar, because the Temple's fiscal year started in the following month: Nissan.
The money collected from the sheqalim was also used for the maintenance and the general expenses of the Temple. But not for its construction. As we will read in this week's perasha Teruma the funds for building the Mishkan (the Sanctuary in the desert) and later on, to build the bet haMiqdash was giving as voluntary donations. In other words, the communal finances depended on a system of donations combined with a fixed flat tax (a sort of a membership fee) that was collected on the month of Adar.
To remember the collection of the sheqalim 1. We read perashat sheqalim one Shabbat before rosh chodesh Adar (that was last Shabbat, February 18th) and 2. We give a fixed a flat donation, only as a symbolic act, to remind us of the machatzit hashequel.
This is an image of the half sheqel coin from the time of the Second Temple (approximately, year 50-60 of the common era)
On the left side one can read in ancient Hebrew fonts: sheqel Israel.
On the right side Yerushalaim haqedosha ( Jerusalem, the Holy)