Northeastwards means towards the northeast — the same idea as northeastward. You mostly hear it after a verb: they walked northeastwards; the front is moving northeastwards. British English likes the extra s (like towards, afterwards, forwards). American English usually drops it and says northeastward. The meaning does not change. Though actually, when the word sits in front of a noun, even British writers often prefer the form without s: a northeastward path, not usually a northeastwards path. So the -s form is especially the “which way are we going?” word. You can still just say northeast in everyday talk.