The phrase "Net 10th Prox" means that payment for goods or services received is due on or before the 10th of the following month. This is a relatively common term of payment utilized by companies in the United States.
"Prox" is short for proximo, from the Latin phrase "proximo mense," which means the next month. Prox terms are typically utilized by vendors when they have specific monthly financial goals when cash must be available, such as payroll on the 15th or the 30th of each month. Another common application of prox payment terms is "Net 30th Prox," which would mean payment was due in full on or before the 30th of the following month. Discounts can also be incorporated into prox terms. For example, "2% 10th Prox Net 30" means a 2 percent discount can be taken if the bill is paid before the 10th of the next month; otherwise, the full amount is due within 30 days.