It means work in the IT sector, have a job connected to IT or computers.

to further add detail, it means that someone works with computers and the technology surrounding computers. The abbreviation of IT means Information Technology. i hope this expanded on the previous two replies

Hi. It means they work in the IT industry. So they could be a computer programmer and help desk worker or anything related to computers and computer programming :)

To work in a field related to computers or IT

Prepositions are so tricky...someone else here pointed out that working "with" computers is slightly different, and working "on" computers is also different (means you are fixing computers).