The Rama (429:1) brings a custom to distribute wheat to the poor before pesach; this is known as maos chitim (money for wheat) or kimcha d'pischa (Passover flour). The Mishnah Berura (429:3) says this is an ancient custom from the times of the Gemarah.
Why is the obligation only on Pesach and not the other holidays of the year, like Sukkos and Shavuos?
Does one have to give wheat or flour, or is money fine?
The Shar Hatzyion (429:10) explains that Pesach is a time that we sit as free men with our families and speak of our redemption from Egypt; if the poor won't have food for Pesach, they won't be able to recite it with joy.
Furthermore, since chometz is prohibited on Pesach and matzos are difficult and expensive to obtain, people will go hungry if we don't provide them. Therefore, there's a special obligation to ensure people experiencing poverty have food for Pesach.
While earlier sources describe the custom to give wheat flour (Mishnah Berura 429:4) or even matzah (Kaf HaChaim 429:12), these days, the custom is to provide money (Chut Shani Pesach: 1:2, and Moadim Uzmanim 7:168).