From PortlandAfoot

Grade, in the world of transportation, usually refers to the level at which a right of way sits. If a path, road or track is grade-separated, it lies at a different height than other nearby objects.

Subway tracks, for example, typically sits beneath the level of the platform. The Hawthorne Bridge bicycle path is grade-separated from the roadway but not from the sidewalk used by pedestrians.

In most of the city, MAX tracks sit at street level and are not grade-separated. This makes it easier for people and other vehicles to walk into the train's path without noticing, which in turn obligates MAX to move more slowly than grade-separated rail lines such as WES.