Can somebody explain head pressure to me?

It is the weight of the water pushing back against the pump or valve. It can be mitigated with a check valve to protect the pump.

"Head" is another term for pressure. It is measured in "feet of head" where 1 foot of head = .433 psi (for water). It's based on the weight of a column of water where the normal atmospheric pressure of 14.7 psia is equal to the force per sq inch exerted by a 33.9 foot high column of water.

The term is used in reference to pumps in a variety of ways such as Net Positive Suction Head, Dead Head, and just Head Pressure. NPSH is the pressure that must be supplied to the suction of the pump for it to operate properly (without cavitation). Dead Head is the output pressure with the pump discharge isolated (zero flow condition), and is the highest pressure the pump will output for a given suction pressure. Head Pressure on a pump curve will show the output pressure for a given flow rate (lower pressures yield higher flows for centrifugal pumps).

Hope this helps.

So what I'm wondering is how much suction a shallow well pump has before buying one. any hints given they don't usually say on ebay. 25' to bottom of well. 9 feet of water.
just

It all has to do with the pump characteristics of the given pump -- take 33' that is a given (atmospheric presure) and start deducting from there. elevation is measured in feet so that is the easiest to take off -- there is 25 feet to the bottom however we do not know exactly what the level of the water is while you are pumping. I would guess that if the well has a reasonable gallonage and you are pulling out within that range your water level to be 20 feet (guess) . Then you have to figure friction loss through all the pipe and fittings on the suction side(also measured in feet) on that short of a set, if the pipe and foot valve are of adequate size i usually figure 10'. All straight centrifugals will pull a max of 33' (minus all the deductions) So technichally it will work to pump some water -- how much is determined by the pump curve at different gallonages ( this is what they cal npshr or what the pump eats and that varies at different gallonages). Jet pumps work on a different theory -- they are still a centrifugal pump however they use a portion of the pressure and gallonage they pump to induce water into the suction line which increases it abillity to suck. -- they are not very efficient so it takes a little more hp to develop the same performance as a straight centrifugal. however they can be used in much deeper wells -- 60-80feet not uncommon. and there are differences in jet pumps (they have there own curves too). A little more info about your specifics would be helpful, if you would like to pm me we can discuss further -- i might even be able to find a pump curve if you have a specific model you are looking at (been doing well work for about 30 years). Or give me a call 503-437 4752 -- dave. I am heading out for the afternoon now so i may be a little slow in my responses.