Your kneecap (patella) sits over the front of your knee joint. As you bend or straighten your knee, the underside of the patella glides over the bones that make up the knee. Strong tendons help attach the kneecap to the bones and muscles that surround the knee.
NETWORKDAYS.INTL(D3,E3,0000000) calculates whole days between the two dates (no weekends, no holidays). NETWORKDAYS(D3,E3,0000000,$A$16:$A$24) calculates whole working days days between the two dates (Sat/Sun weekends, holidays as per your list in $A$16:$A$24). Difference in non-working days between the two dates.
We can tell there are 31 days between the two dates. This is a very simplified example and in most professional worksheets, you will not perform calculations on dates in this way. Instead it is more likely that you will use the DATEDIF function.