The file \index.html\ is commonly the first file a web server loads when someone visits a directory on a website.
That is why it often acts as the front door of the site.
Why it matters
If a directory contains an \index.html\ file, the server will usually show that page instead of listing the folder contents.
Common equivalents
Depending on the technology used, the main entry file might also be:
- \
index.php\ - \
default.aspx\ - framework-specific entry points
The idea is the same: it is the file the server expects to use as the default page for that location.