The sources of the MOSFET are floating. THey have to be like that to not ruin operation of the circuit. For example, rIght now, if a FET turned on for whatever reason you would get a short-circuit straight to ground.
Imagine we placed connected an isolated/floating (as in not connected to anything else except the things I'm about to say) voltage source to between gate and source with the negative pin of the voltage source connected to the battery and positive pin connected to the source. If we say made the voltage source 0V (a piece of wire basically) the FETs would stay off right? And if the voltage source was made something like 5V or 10V the FETs would turn on, correct?
Okay, now we can make such an isolated voltage source a few ways...we could use isolated DC-DC converters that use transformers so the input ground does not have to be the output ground...big, bulky, expensive, complex, overkill for this application not to mention there are some "gotchas" regarding FET switching noise travelling backwards into the converter and disrupting it, among other things. We could also use transformers directly with and other methods...
What if we used a battery for the voltage source? Great! But now we basically can't turn it off (or if the battery is dead we can't turn it on). What if we made a completely separate circuit completely powered by the battery and the ground of this circuit would be connected directly to the ground source of the MOSFET. THis circuit would accept inputs and send out a voltage into the gates so we could turn the FETs on and off. All voltage outputs from this circuit would be with respect to the source. So a 10V output from this circuit would be a 10V relative to the source of the FETs. Stick that into the gate and voila! It turns on. This circuit is considered "floating" because it's "reference voltage" isn't fixed- it's not relative to ground. It moves up and down with the FET source voltage.
A bit extravagant right? Again, we have a whole extra power supply (the battery) dedicated to the gate. What can we do? What if we charged up a capacitor to the required gate-source voltage difference needed to turn the FET on (to turn it off we can always just short the gate and source pins producing 0V difference- that's easy.). It turns out, we can. Better yet, we can do it with circuitry that uses the system ground as it's reference. What we basically do use switches to connect the + and - of the capacitor across a ground-referenced power supply and let it charge up. THen we disconnect both capacitor terminals using the switches leaving a 10V charged capacitor that has no pins connected to anything. We use a third switch now to connect the NEGATIVE pin of the capacitor to the SOURCE pin of the FET. Suddenly, the voltage on the postive pin of the capacitor is "Source Pin Voltage relative to ground+10V". If we connect the positive pin to the gate, we suddenly get 10V across the gate-source pins and our FET can turn on.
THis is bootstrapping- the IC uses it's own voltage to lift itself to a higher level sort of like pulling yourself out of quicksand by bending over and pulling yourself out by tugging on your boot's straps. Use an IC (you are looking for a high-side gate driver IC). from the resources I posted. THat's just the concept of how it works. There's a lot of crap inside those ICs to actually pull off this off. Pick an IC that meets your requirements and the datasheet will have the schematic.
BTW, the diode on the ICs are there to only allow charge flow from Vgate_drive_supply into the capacitor but not the other way around. That's because when the capacitor's negative pin gets connected to Vsource the + pin of the capacitor is going to be Vsource+Vgate_drive_supply above ground, which would then cause the capacitor voltage being used to drive the gate to drain back into V_gate_drive supply. THe diode only lets charge flow from Vsupply into the capacitor, and the charge from there can only ever flow into the gate.