What does 'And so at that tyme Sir Launcelot had the grettyste name of ony knyght of the worlde and moste he was honoured of hyghe and lowe' mean?

In today's English it means:

And so at that time Sir Lancelot had the greatest name of all knights of the world and he was the most honored by the high and low.

Which means that Sir Lancelot earned the title of Greatest Knight of the World and all the people both great or small, rich or poor, mighty or weak gave him the most honor above all other knights.