In laments terms, it means the coach at XYZ has given an offer to a kid to come play baseball at their college. Nothing is in writing and nothing is official until they sign the NLI as TPM said. In this 1-4 year gap the player may not improve like the coach thought, the coach might leave and the new guy doesn't honor the agreement, the kid can just give up baseball without a penalty, etc...in other words a million things could happen. The further out from graduation the kid is, the more likely one of these things will happen.
In the meantime the kid has tweeted and posted and informed PG that he's thrilled to continue his education and baseball career at XYZ university. This means that ABC university doesn't offer the kid and usually won't even look at the kid again. So in August of the kids senior year when coach XYZ says "Sorry kid, it's not going to work out, we are pulling your offer", the kid is up a creek. No one is following him anymore, and most high dollar spots are filled.
While there is a certain thrill to committing early, I have seen a wave of very high 1%'ers waiting. They are waiting to see if that school/program really might be the best choice for them. They are waiting until the kid is a little older and can make a more informed decision other than "I like the school colors", which is honestly something I heard come out of a kids mouth! So to sum up, sounds cool, but might be a huge mistake.