The tone when telling someone to 'shut up' implies annoyance or frustration; it can be offensive. Enough noise, just shut up now!
Pro Tip 2/3
Cultural Sensitivity
'Shut up' is often considered impolite and may be seen as disrespectful in many cultures. In class, he told his friend to shut up and the teacher got upset.
Pro Tip 3/3
Non-literal Use
Sometimes 'shut up' can express disbelief or amazement, not always literally meaning 'be quiet'. You got the job? Shut up, that's amazing!