The letter dalet represents dimensions and relations.
The letter dalet is based on a glyph of the bronze age alphabets called dalt "door" (door in hebrew is delet), ultimately based on a hieroglyph depicting a door.
The dalet hints to dal, poor, who knocks at the door and begs for alms.
In the exposition of the Talmud, the letters gimel and dalet stand for gmol daleem, be good with he who is in need. (source: Shabat 104a).
But the dalet turns its back to the gimel. This is because the needy must not face their benefactors. In the highest form of charity neither the benefactor nor the beneficiary must know each other's identity. (source: Shabat 104a).
In the Temple there was a special room called lishkat hassaym, the chamber of silence, which anyone, rich and poor, could enter, but only one at at time. Those who could, would make an anonymous contribution. Those who were in need would discreetly take from it.
The giver would not know whom he was giving to, and the receiver would not know whom he was receiving.(source: Shkalim 5:6)
I judaism the letter dalet, along with the he (and very rarely gimel) is used to represent the Name of God.
A good example is the keter (crown) of a tallit, which has the blessing for donning the tallit, and has the name of God usually represented by a dalet.
A reason for this is that the he is used as an abbreviation for HaShem, "The Name" and the dalet is used as a non-sacred way of referring to God.
The movie below shows how I make a clay prototype of a hebrew letter. ==========