A Web site is a group of related Web pages. Most Web sites have a home page as their starting point. The home page is interconnected with other pages by using hyperlinks. You can use top-level sites and subsites to divide site content into distinct, separately manageable sites. Top-level Web sites can have multiple subsites, and subsites themselves can have multiple subsites. The entire structure of a top-level Web site and all of its subsites is called a Web site collection.
This hierarchy enables users to have a main working site for the entire team, plus individual working sites or shared sites for side projects. Top-level Web sites and subsites allow different levels of control over the features and settings for sites. The administrator of the Web site controls the ability to create, access, and contribute content to a Web site.
Click on Settings Button and then select Site Contents link to go to the Site Contents page.
Here if scrolle down you will find New Subsite link under SubSites lable. Click on New Subsites to create sub site.
Specify the Title, Description, and Url Name, etc. Choose the blog Template for this subsite. Also, choose other options like permissions and navigations as you desire. Click on Create button.
Now This is your Sub site, here you can find your sitcollection name and subsite name like bellow