The Line Simplification menu item is used to merge polygon, polyline, and region segments together. The general idea is to eliminate points that do not add meaning to the overall geometry. For example, a polyline that is a straight line needs only two points to describe it; additional intermediate points would be discarded.
This is an inplace operation; existing geometry will be updated and no new geometry will be added.
There are three algorithms to choose from:
DouglasPeucker:  This is a very commonly used algorithm, and is also known as the "iterative endpoint fit algorithm" or the "splitandmerge algorithm". The general concept is to identify suitable line segments, and simplify each one using this technique. Points are then identified that must be kept, and lines are drawn between these points to identify additional points that fall outside a specified tolerance (and thus must also be kept). Any point that is below this tolerance is discarded. The process is repeated recursively until all points are tested. 
Vertex Reduction:  This is a very simple algorithm. Given a section of geometry, it identifies adjacent points that are closer together than a specified tolerance. It then removes one of these two points, and the process is repeated. 
Remove Duplicate Points:  This algorithm is actually a "preprocessor" to all the above algorithms, but may also be useful by itself. Its only function is to identify adjacent identical points, and remove the duplicate. Duplicate points are not desirable in a good model. 
