What does $"2/15"$ mean, with respect to "credit terms of $\frac{2}{15},\frac{n}{30}"?$

An interest of $2\%$ will be allowed if invoice is paid within $15$ days from the date of invoice.

A discount of $10\%$ will be allowed if invoice is paid within $2$ days from the date of invoice.

An interest of $10\%$ will be charged if invoice is paid after $2$ days.