A connective phrase meaning 'just at the time (when)...' or 'thereupon/shortly thereafter', used to link an action to another event that happens immediately after.
家に着いたところへ電話がかかってきた。
Just as I arrived home, the phone rang.
彼が仕事を終えたところへ上司が急に訪ねてきた。
Right after he finished work, his boss suddenly came to see him.
お弁当を広げたところへ雨が降り出した。
Just as we opened the lunch, it began to rain.