This option allows the user to track nodes, whose traces are displayed. For a single mesh for the whole analysis, the node will follow the path of the deformed mesh across all steps. When several meshes are used for several time steps, then the nodes can follow the track of the original mesh or from the deformed one.
Go to Preferences --> Postprocess --> Stream lines to set and manage stream lines options, like the colours colors used, style which with they are drawn, etc. (see Stream lines)