voltage
Voltage is the push that drives electric charge around a circuit. The cleanest picture is water in pipes: voltage is the water pressure, the difference in height or pump force that makes water want to flow downhill. No pressure, no flow; more pressure, harder push. Crucially, voltage is always a difference between two points, like saying one shelf is 30 cm higher than another. A 9 V battery means its plus terminal sits 9 volts higher than its minus terminal.
Formally, voltage is energy per unit charge: one volt means one joule of energy is given to each coulomb of charge that moves through. The symbol is V and the unit is the volt (also V). Because it is a difference, you always measure voltage across something, across a battery, across a resistor, across a lamp, by touching two points. Common values: a single AA cell is 1.5 V, a USB port is 5 V, a phone battery about 3.7 V, household mains 110 to 240 V, which is dangerous.
Voltage by itself does nothing visible; it is potential, the readiness to push. Only when you connect a path does charge actually flow. A 9 V battery sitting on a table has full voltage but zero current. This is why you can safely hold a 9 V battery but never household mains: it is the combination of voltage and what it can push through you that hurts. Voltage relates to current and resistance through Ohm's law, the first real tool of circuit work.
A voltage divider of 10 kilohm over 5 kilohm across a 9 V battery puts 9 times 5/(10+5) = 3 V at the junction; the bigger share of the push falls across the bigger resistor.
Voltage splits up among parts in a loop in proportion to their resistance.
There is no such thing as the voltage at a single point until you pick a reference; we usually call the reference point ground and treat it as 0 V, then measure everything relative to it.