Even with the most careful approach to 3D modeling, errors are bound to happen. Most errors and warnings can be fixed. In the design timeline, yellow indicates a warning. This means that Fusion 360 is missing some information but it remembers what the information probably is and makes its best guess so the model keeps working. These should be inspected and fixed. Often the order of operations is the cause of the problem or perhaps a plane changed shape so an extrusion doesn’t know what profile to select. Right clicking on the highlighted feature on the timeline will give more information about what the problem is.

Red highlighting indicates an actual error or failure to compute the model. These usually need to be fixed in order to proceed. Right clicking on the timeline to inspect the error is the first step. Sometimes it is as simple as re-selecting a mirror axis or an edge for a fillet. Other times the error is more catastrophic such as the result of an earlier sketch not being fully constrained. This video  walks through fixing some errors on the design history timeline in Fusion 360.