British tabloids have a strange "tradition" of including a page amongst the news with a photograph of a topless woman. In The Sun newspaper, probably the best-selling paper in the UK, the image tends to be on page three, the first page one flicks to after opening the cover, hence the moniker.
Many women find it offensive. Imagine having to sit on the Underground whilst a man sitting next to you is ogling a naked woman.
There are regular campaigns to try to end this practice, and this is just the latest one to ask The Sun to get rid of "Page 3" girls. Hopefully one day one of the campaigns will work.