Is this file supposed to be a new legal file required to comply with world law where its contents describe the rules for my website usage, or does it belong to a special content management system?
License.txt is simply a generic name for any license file. There is no web "standard" or convention for "license.txt" files like there is for, say, robots.txt or humans.txt. However, many web "applications" (those written in scripting languages, for example) will use a "license.txt" file to convey the terms under which the application is distributed (common to open source projects especially). This file often resides in the root of an application directory.
As pointed out in the comments, retrieval of the "license.txt" file can give various kinds of clues about the system that provides it that may be of use to someone who wishes to exploit the site, even if this information doesn't specifically relate to "license.txt." That said, simply having a "license.txt" file doesn't mean your system is vulnerable.
There is no reason to keep this file available (or any non-essential file after setup for that matter) and it should be removed to discourage bad behavior and thwart any real exploit attempts.