standing in front of앞에 서다 means “to stand in front of” or “to stand before.” 서서 connects this action to what follows, meaning “standing in front of and then...” or simply “while standing in front of.”
KO: 선생님 앞에 서서 큰 소리로 발표했어요.
EN: I gave my presentation in a loud voice while standing in front of the teacher.