Safari is Apple's native browser, not the operating system, the latest of which is OS Sierra 10.12.1. So you can update Safari and stay with El Capitan.
Usually we are only offered minor updates to Safari. You only get major upgrades to Safari along with an upgrade to a major new version of the MacOS.
So it's not surprising that you are confused. This is a major upgrade being offered without the need to upgrade your OS version.
Safari 10 is a really worthwhile upgrade. It blocks scareware/ransomware (i.e. it keeps malicious Javascript from being able to trap you on a Web page), it uses HTML 5 by default instead of Adobe's Flash (the later being a frequent vector for malware), and it includes several other important security measures. This increased security is so important that it is probably why Apple decided to make Safari 10 backwards compatible with El Capitan and Mavericks.
http://tidbits.com/article/16783
On caveat. If you update to Safari 10, or you get it by upgrading to MacOS 10.12 (Sierra), you may find, to your dismay, that a number of Web sites don't seem to work. The reason is that Safari 10 disables Flash by default, and not all Web sites offer an HTML 5 alternative yet. This is easy to deal with once you know how.
http://apple-support.applehelp.support/safari-10-flash-is-disabled-by-default/
"Like Google and Microsoft Windows, Apple will be de-emphasizing (reduce the importance of) Adobe Flash in its first-party web browser Safari. Safari in Apple Mac OS Sierra has Flash turned off by default. If you want to use flash you have to turn on Flash for each website you visit and grant it authorization to work just once or every time you visit.
"Most websites that detect that Flash isn’t available, but don’t have an HTML5 fallback, display a ‘Flash isn’t installed’ message with a link to download Flash from Adobe. If a user clicks on one of those links, Safari will inform them that the plug-in is already installed and offer to activate it just one time or every time the website is visited."
Hopefully this will save you from some aggravation.