The main difference between the USA's "first inventor to file" and the "first to file" systems in most of the rest of the world is how disclosing the invention publicly by printed publication, use, or sale before you apply for a patent affects patent eligibility.
In the "first to file" systems, you lose eligibility. In the "first inventor to file" system of the US, you have a grace period of one year from such disclosure before you lose eligibility, just like you did under the prior "first to invent" system.
To a decent first approximation, the change to the US system mostly just affects how things are handled if independent inventors try to patent the same thing.
Under the "first to invent" system, the patent went to whichever applicant invented the thing first. But when is something invented? When you first think of the idea? Or when you have reduced the idea to practice? Or sometime in between?
Under the US first to invent system, it was when you conceived the invention if you then worked diligently from that point to reduction to practice. If you stop so working and resume, your lose the prior invention date and your new invention date is when you resumed working diligently on reduction to practice.
When two or more inventors were claiming the same invention determining who was actually first under those rules could be difficult and messy, requiring hearings, and costing the inventors money for attorneys. Now with first inventor to file, we pretty much don't care who thought of it first--we just care which person who independently thought of it reduced it to practice and submitted their patent application first. That's a lot simpler, and requires no expensive hearings.
