It's a set of software that basically includes a word processor (for writing documents), a spreadsheet (for calculations), and a presentation application (for slideshows). There are other programs included but those are the core ones. If you don't think you will need to do any of those with your computer like if you will just use it to play games, you don't need to install MS Office.
Microsoft Office is a tool where you could create documents, presentations, and spreadsheets for data, events, or just simple fun. It is crucial, in my opinion, for a job requiring skills in this, or school life.
It is MS Word, Excel, Publisher, Outlook, etc. It is what we use to produce documents, spreadsheets, and email. I would find it very difficult to function without it. I guess it depends on what you use your computer for.