That is a site dedicated to a rather specialized sub topic, a general photography site would talk about all kinds of photography, a child portrait niche site would only discuss kid photography.

The thinking is, you will be able to get search ranking much more easily for a specialized niche site. Many sites rely on search engines for traffic, where about 90% of all traffic comes from the top 10 results on page one, there is little chance of your site being seen near page one when competing with many thousands or millions of results,

Niche website is a small kind of website dedicated to only a particular topic, rather a micro topic.

Niche sites mean relevant or categorized sites. If you have a real estate website then you must clear that all backlinks coming from relevant real estate sites ( niche sites). You get quality backlinks & traffic.

Niche site means categorized sites. For example you own a health site and your all backlinks are coming from health related sites then these sites will be niche for your site.