#The earth

Possible meanings are 1) this is a metonym, AT: "The people who lived on the earth," (See: [[:en:ta:vol2:translate:figs_metonymy]]) or 2) "The earth itself."

#was corrupt

The writer speaks the way he would speak of food that has become rotten. AT: "was rotten" or "was completely evil"

#before God

Possible meanings are 1) see how "the presence of Yahweh" is translated in [[:en:bible:notes:gen:04:16|4:16]] or 2) "in God's sight."

#and it was filled with violence

The writer speaks of violence as if it were something that could be put into a container and of the earth as a container. AT: "there were very many violent people on the earth" or "because it was full of people who did evil things to each other"

#all flesh

"all physical beings," everything with a body, including humans and animals

#had corrupted their way

"they stopped living the way God wanted" or "they behaved in an evil way" (UDB).