It means you pay the first $1000 of your annual medical expenses(not including dental, that is a different deductible) before your insurance pays a dime.
Every January 1 the deductible resets.
Example, January 1 you have paid nothing towards your deductible.
January 2 you go to the doctor and are charged $250. You have to pay the $250 out of pocket, and your deductible is reduced to $750.
You get lab tests, those cost $500. Again, your deductible is not satisfied so you have to pay the $500 out of pocket, your deductible is now reduced to $250.
You go to the doctor again and you are charged another $250. Again you have to pay the $250 out of pocket, but now the deductible is reduced to $0.
You go the doctor again and are charged $250, this time the insurance pays it.