In broad and confusing terms, a mechanical linkage is “an assembly of systems connected to manage forces and movement”. But you can essentially think of it as links (bars) that are connected to each other in order to transfer force and motion where it is needed. There is always at least one link that is stationary (fixed), and the movement of the system is based on the moving links attached to the fixed point.
An example of a linkage in daily life would be a pair of scissors. Each blade represents a link and the fixed joint in the middle allows the scissor to open and close. Other examples of linkages in real life would be scissor lifts, umbrellas, or even a folding clothes rack.
Linkages can be really useful on our robots, as we can use them to lift large objects, reach tall places, and extend mechanisms outside of the frame perimeter.
Some common linkages examples include: