What you have to write will vary depending on what your discipline is.
But in general terms, it is a document that sets out the background to your research question, what exactly you are proposing to study and how you intend to research it.
In my area, you would start with an introduction naming the issue. You would then have a background section in which you set out the existing literature, and highlight the gap that is the area your PhD will fill. Then you would have a methods section detailing how you propose to investigate the issue. This needs to address any ethical issues as well.
It is also helpful to add a timetable for the work.
I used Google, typing in several variations on 'PhD research proposal'. Lots of university websites have guidelines and helpful stuff on their websites, so I started with the university I wanted to apply for and other UK universities with a good track record in my subject. I copied and pasted info from those sites into a word document and kept it open whenever I was working on my proposal.
I got the second attempt accepted (would have had the first accepted but I forgot about referencing after many several years out of academia!)