IT stands for "Information Technology". This refers to an industry that specializes in setting-up, maintaining, and supporting computer hardware/software for their customers (typically workers in an office environment). If there is someone in your office or school that you call for help when you have an issue with your computer, then they are your "IT expert".
IT : Information Technology; (capital letters because they are the initials).
The term covers everything to do with the use/application of computers, particularly (but not exclusively) in a business context.
At one time, many different expressions were used : "Computing", "EDP (Electronic Data Processing)", etc. As computers became increasingly used for applications other than pure calculation and data manipulation, the name "IT" became virtually a standard.
More recently, "IS" (Information Systems) has gained ground.
Maybe others can tell us if they know of further evolutions in the terminology.
IT - was thought to place too much emphasis on the technology and hardware when it was often the systems that needed working on - how work flows through an office and how the technology is used hence IS.
Then e-mail came along, and the internet and web pages and internet telephony and the Communications bit got added. What children learn at school in the UK now is ICT - they don't just learn how to use a computer. They are given problems that they have to solve using hardware and software and the internet - it would be nice to think that they also learnt how to communicate about their solutions to people less computer literate but I don't think that is always the case