What does the words his legs bestrid ocean reared arm crested mean?

Bestrid - (verb) to have one leg on each side of (a barrier, river, etc.)

Ocean reared - refers to the giant being (likely Atlas or Chronos) who emerged/rose from the ocean.

Arm crested - refers to the summit or peak of a wave/crest.

So the phrase means: The giant has one leg on each side of the ocean, his arms are raised and he is holding the weight of the sky on his shoulders.