It separates two lanes traveling in the same direction
It means that lanes to both sides of that line travel in the same direction.
It means that you may change lanes freely (i.e.: pass another vehicle) from one to the other.
no passing
In the United States, it means "you may pass."
A continuous white stripe parallel and to the right of your lane means that you are at the far right of lawful driving on the highway. This line is also called a 'fog line', as it helps the driver see where the edge of the highway is when it is foggy.
You are to remain in your lane, and traffic in adjacent lanes moves in the same direction.
A double solid white line separates two lanes of traffic going in the same direction. Crossing a double solid line is prohibited.
you can pass a vehicle in front of you if there is no oncoming trafic
Depending on which country you are in. Generally it means 'Do not cross', keep to your your lane, no passing.
divided highway
Double solid yellow is between two lanes of traffic with oppsing directions. This indicates no passing a vehicle that is traveling the same direction by moving to the oposing lane momentarily. You have to stay in your lane. Solid white is a line between two lanes going the same direction, but indicates changing lanes is not admissable unless safety concerns exist. Again, you have to stay in your lane.
no passing