The dalet is the fourth letter of the alef-beis. The Talmud tells us that the dalet represents the poor person. Thus the phrase gomel dalim: the benefactor who gives to the beneficiary.
The Talmud also tells us that when we observe the shape of the dalet, its single leg stretches toward the right—in the direction of the gimmel. This teaches the poor person that he has to make himself available to receive the charity of the benefactor. Similarly, the small extension on the right-hand side of the dalet’s horizontal bar looks like an ear, for the pauper must always be listening for the presence of the wealthy man. However the left side of this bar doesn’t confront the gimmel, the giver, but faces left, toward the letter hei, which represents G‑d. This instructs us that we must give charity discretely and not embarrass the poor person. The pauper must put his faith in G‑d, Who is the ultimate Giver of the universe.
The Mishnah tells us that in the Holy Temple, there was a room called “the Silent Chamber.” One would enter this room alone and close the door directly behind him. In the room was a big box. One had a choice: either to put money into the box or to take some out. Of course, the rich man would put money in. And after him, also alone, would come the poor man, who took money out. It was all done discreetly. The rich man couldn’t see to whom he was giving charity. The poor person didn’t know from whom he was taking it.
A second approach to the form or design of the dalet is that the dalet represents a doorpost and a lintel. The vertical line is the doorpost; the horizontal line is the lintel. What is the connection between the door and the poor man? Customarily, a poor man must knock on doors.
There’s also a third interpretation provided by the teachings of Chassidus. This view points out that the dalet is composed of a reish and a yud. What’s the difference between the dalet, ד and the reish, ר? A yud. If one affixes a yud to the upper right-hand corner of the reish, the reish becomes a dalet. The yud, a very small letter, represents humility. That humility is what separates the reish from the dalet. The mezuzah on our doorposts contains the famous paragraph of the prayer known as the Shema. In the Shema we say, “Hear O Israel, G‑d is our L-rd, G‑d is One.” The word echad, one, as in “G‑d is One,” is spelled with the letters alef, ches, dalet, אחד. What happens if the yud is removed from the dalet and it becomes a reish? The word is no longer echad, but acher, אחר—other. If such a mistake were made, this would now translate into, “Hear O Israel, G‑d is our L-rd, G‑d is other (i.e., other gods).” So critical is the aspect of yud, humility, in the belief in G‑d’s oneness that its omission might cause one to reject G‑d and believe in the existence of other omnipotent powers in the universe. The Midrash tells us that if one switches the reish for the dalet, he’s destroying all the worlds.
Source: Secrets of the Hebrew Alphabet