What is the lever rule in a Txy diagram

Sinistar9

I dont understand what the lever rule is on a Txy or Pxy diagram

I know that when you are given a mole compostition of said component, you go up to whatever temp or pressure is given to see the compositions.
but, if it is inside the 2 lines of vapor and liquid, you use the lever rule.
can someone plz explain this? thank you

Sinistar9

bump