Here Moses speaks of a person promising to do something as if his promise were a physical object that he ties to himself. AT: "committing to fulfill a promise" or "promising to do something" (See: [[rc://en/ta/man/translate/figs-metaphor]])
These two sentences have similar meanings and are combined to emphasize that he must fulfill his promises. Here "word" is a metonym that refers to what he said. (See: [[rc://en/ta/man/translate/figs-parallelism]])
The phrase "his word" refers to a man's oaths and vows. Here Moses speaks of not fulfilling these as if they were physical objects that the man could break. AT: "he must fulfill his promises" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-metonymy]])
Here the word "mouth" is a metonym for the things that the man says. AT: "to do everything that he says he will do" (See: [[rc://en/ta/man/translate/figs-metonymy]])