If you stay as an Insider running Insider Preview builds you will have an effectively free Windows 10 Pro for as long as the program runs (there's no hint of it ending any time soon). This has no link to any existing underlying license.
Each individual build will expire, but as an Insider you will keep getting new builds so you will stay active. If you drop out of the program you stop getting new builds, meaning the build you are on will eventually expire and you will have to seek out a proper license.
The conditions of the Insider program will remain as they are now. It's pre release software, zero warranty, stuff might not work, and the logging (which is nowhere near as draconian as people think it is) will remain. Basically if you agree to be a tester for Windows Updates you get to run test versions of Windows for free.
If you don't want this, and want just stable final proper Windows then you need a license.