"Daleth" is the name of the fourth letter in the Hebrew alphabet. Each of verses 25 through 32 begins with this Hebrew letter. See "Formatting in this chapter" in [Psalm 119 General Notes](../119/intro.md).
This is an idiom. Possible meanings are that 1) he thought that he would die soon, or 2) he lay in the dust because he was very sad, or 3) he realized that he desired the worthless things of the earth. (See: [[rc://en/ta/man/translate/figs-idiom]])