Electric current flows in a circuit from the positive to the negative. It is often portrayed as a flow of electrons but it's more of a sloshing back and forth and a transfer of energy. In electric panels you have hot wires (like the positive pole of a battery) and neutrals which return the flow of energy to its source at the power plant. The neutrals are the white coated wires above. Ground wires (the bare ones) are there to bleed off any excess current on a fixture, switch or appliance so people don't get shocked. In the panel above the neutral bus bar has a green screw at the top. The neutral bus bar has a silver screw. We can see that given the layout it is difficult to access the grounded bar. So maybe that's why most of the grounds and all the neutrals are jammed together on the neutral bus. My understanding is that if you connect neutrals and grounds there is a possibility that a ground wire can get energized, making the fixture it's attached to dangerous. However, the rules also state that the neutral bus bar and the grounding bar must be bonded in the main panel (the silver bar at the top connecting the two). This apparent contradiction likely has to do with electricity following the path of least resistance so if all the grounds are on one bar and neutral on the other, the current will go where its supposed to. Whereas if a neutral and ground are directly connected it has a another choice.