Some models have overhanging parts, which means that part of the model floats in the air when you would print it. In this case you will need support to prevent the plastic from falling down.
In Cura you can either select “Touching buildplate” support or support “everywhere”. With touching buildplate support Cura will generate support at the places where it can reach the floating parts from the build plate. Support everywhere means that support can also be placed on or inside a model.
The overhang angle defines how much overhang the model should have for support to be printed under it. By default this is set to 60 degrees, which means that support will be printed for parts that have an overhang of at least 60 degrees. Changing this setting to 0 will lead to support on all part that hang over more than 0 degrees and an overhang angle of 90 degrees will create no support.
Since Cura version 15.06 support generation has been improved; using towers is one of these improvements. If this option is enabled, specialized towers will be printed to support tiny overhang areas. These towers have a larger diameter than the region they support and are smaller at the top. This way there's only little contact with the model, so that the visible traces of the support structure are decreased.