Key date coins are simply coins that are valuable based on the dates and/or mint marks. For example a key date for a penny would be the 1909s VDB or a 1982 no mint mark dime - both of these are valued at several hundred dollars and over 1000 in really good shape.
Any coin book will give you the values of all the coins, but you will have to sift through all the years to pick out the really valuable ones with key dates.
My experience in pulling out coins from the ground is this - even if I found one of these, it would not be worth much as coin values are based on condition and the ones I get that have been in the ground for years are always in bad shape and nearly hard to even see date on some.