'Upsetting content' warnings at universities: a harmless courtesy or oversensitivity?

Some higher education institutions are accused of 'wokery' and pandering to 'snowflake' students because texts now sometimes come with notes saying some students may find the content distressing. People in favour of such warnings see them as an unintrusive way of respecting students’ mental well-being. But what does 'triggering' actually mean? What do students think about trigger warnings? How did they end up on university campuses? And do they actually work?