Lead lag is caused by the blades speeding up and slowing down as they flap up and down.
The blade on the advancing side experiences an airflow equal to it's speed plus the forward speed of the helicopter. The blade on the retreating side experiences airflow equal to the speed of the blade minus the forward speed of the helicopter. This causes dissymmetry of lift.
To counter this, the blades can flap up and down around the flapping hinges. The blade on the advancing side flaps up, reducing the angle of attack and therefore the lift generated. The blade on the retreating side flaps down, increasing the angle of attack and therefore increasing lift.
See this video at around 1:30 to see flapping in action. This is a gyrocopter, but you can really see the blades flapping on this one at 0:39 and 1:17.
Due to conservation of momentum, the blade flapping up speeds up as it's centre of gravity moves inwards toward the rotor hub. Since the rotor is turning at a constant speed, the blade must move forward around the lead/lag hinge. In a similar way, the blade flapping down slows down so lags.
The blades will hunt forward and backwards from the neutral position in a sine wave as the rotor rotates. Lead/lag dampers reduce the force of this hunting.
A variation of flapping hinges (independent hinges for each blade) is a teetering hinge on 2 blade helicopters where the hinge is connected to each blade. If one blade rises, the other must fall and vice-versa. The entire rotor disc "teeters" on top of the drive shaft. This is known as a semi-rigid head and as far as I know, is used on all 2 blade helicopters.
In semi-rigid and rigid heads, the overall effect of the blades speeding up and slowing down cancels out in the complete cycle of rotation and other mechanisms are used to reduce the hunting loads. Imagine a 4 blade rigid rotor. 2 blades are trying to speed up the rotor and 2 are trying to slow it down in equal amounts.
One serious side effect of lead/lag on fully articulated heads is ground resonance. Since the blades can move forwards and backwards, it is possible for the blades to get closer together on one side than on the other, especially when the helicopter lands with a roll angle, so one gear hits the ground first. This shifts the centre of balance away from the centre of rotation which can induce a wobble of the rotor head, which translates into a vertical "bounce" vibration at the landing gear. If conditions are right (or wrong, depending on your view point), the resonance can be amplified by the natural resonance frequency of the helicopters landing gear and can lead, if allowed to continue, to the destruction of the aircraft.
If you think that helicopter blades are quite rigid, think again.