# when you went out from Seir, when you marched from Edom

This refers to the time when the Israelites left Edom to start conquering the people in Canaan. 

# Seir

Seir is a mountain on the border of the land of Israel and Edom.

# the earth shook, and the skies also trembled; also the clouds poured down water

This is poetic language that emphasizes Yahweh's power by describing it as causing earthquakes and storms or the people of Canaan being terrified as the Israelites were about to attack them.