This is personification that means that this is what someone has written in the scriptures. Alternate translation: "For we read in the scriptures that" (See: [[rc://en/ta/man/jit/figs-personification]])
Paul is using this quotation as a metaphor meaning that church leaders deserve to receive payment from the Christian community for their work. (See: [[rc://en/ta/man/jit/figs-metaphor]])
An ox "threshes the grain" when it walks on or pulls a heavy object over the cut grain to separate the grain from the stalks. The ox was allowed to eat some of the grain as it worked.