Historical Germanic linguistics: What do they mean by "jo-" and "wa-" stems?

I’ve noticed in anything that deals with the history of Germanic languages, whether it’s Old English, Old Norse, or Old German, nouns will be described as breaking down into various declensions based on the stems. Yet the words described in the examples don’t seem to have anything like those syllables, or even anything that could have evolved from them.

For example, here’s a “ja- stem” from Old English:

           Masculine                             Neuter 

N hrycg (back) here (army) ende (end) cynn (kind) ríce (realm)
G hrycges heriges endes cynnes ríces
D hrycge herige ende cynne ríce
A hrycg here ende cynn ríce
N hrycgeas herigeas endas cynn ríciu
G hrycgea herigea enda cynna rícea
D hrycgium herigum endum cynnum rícium
A hrycgeas herigeas endas cynn ríciu

I apologize for the format; cut and pasted tables don’t display very well. N., G., D. and A. stand for nominative, genitive, dative, and accusative cases, each line gives those inflections for each of the five words shown. But why are these “ja-” stems?

This link, looking at ja- and jo- stems in Gothic might help. I know it’s Gothic, and not old English, but, as it explains: