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A bar or measure is used in writing music. Each bar is a small amount of time. Most music has a regular beat (or pulse) which can be felt. Each bar usually has the same number of beats in it. Music that feels like 1-2-3-4-1-2-3-4 will be divided into bars with four beats-worth of music in each bar.
The bar line (or barline) is a vertical line which separates the bars.
At the beginning of the music there will be a time signature which shows how many beats there are in each bar.
A double bar does not mean "two bars". It means two single barlines drawn close together, one being thicker than the other. It is a "repeat sign". If there are two dots on the left of the double bar line the player should go back to where there was previously a double bar with two dots on the right, or back to the beginning of the piece.
A double barline without dots shows the end of a piece of music.
The first beat of a bar feels stronger than the others. When a conductor beats time, his hand (or baton) always goes DOWN for the first beat of the bar. This feels strong. The last beat of a bar is an "upbeat" because the conductor's hand always goes up.
Barlines have been used since around 1600.