英语问题Serious-looking men spoke to one another as if they were

英语问题
Serious-looking men spoke to one another as if they were oblivious of the crowds about them, and bobbed up and down repeatedly in little bows,as they exchanged the ritual formula of gratitude and respect:"Tomo aligato gozayimas."
bobbed up and down repeatedly 这个是什么意思？bob 在这里又是什么意思？