It means that the trial token will be applied to the javascript context that you are using it in. If you add it in a content script, then it will be exposed in the website's environment as well. You can't enable a feature for just your content script, but not the MAIN world.
Not specifically, no. Trial Tokens are kind of like giving you the ability to turn on a flag in the user's chrome://flags (they don't literally map to individual flags, but conceptually it is similar). It is just a gate to some kind of change in Chrome. It could be a new feature, a change in functionality, or removing a feature. There are even things called Deprecation Trials that actually re-enable a feature that was recently removed.
All that to say, there is no guarantee that it can't break behavior - in fact some may be explicitly breaking something. Trial tokens are intended to trial behavior, and as such code that is using it should be written defensively, because there is no guarantee their effects will be there long term.
patrick