First and foremost, if you were asked to visit campus on an 'Official Visit' by a coach -- congratulations!
Understand that a visit that is 'Official' in nature for a student-athlete is a visit to a D1 or D2 college campus by the student and his/her parents paid for by the college. This means that the school would pay all (or some) of the following: (1) Transportation to and from the college, (2) Room and meals while visiting, (3) 'Entertainment' expenses, typically in the form of a home game for one of their athletics programs (doesn't have to be the sport you're looking to place, though that is ideal).
Typically, D3 schools will not offer an official visit by these standards, but the school can certainly choose to do so. (Those that do are the ones that have larger funding and recruiting budgets).
By NCAA rules, student-athletes can only begin accepting official visit offers on Day 1 of their high school senior year. In the case of baseball, for example, October is THE month for recruiting visits, so the timeline for these 'official visits' begins very soon.
Rule of thumb: If a coach/program offers you an official visit, you are most likely on the upper end of their recruiting list.
Of course, if you have any questions or concerns with the visit, you should always ask the coach you spoke with initially to clarify (what type of visit, what specific things you may be doing on the visit, if you can see a classroom/have a full campus tour, etc).
You'll be asked any number of questions while on your visit, a set of which we've covered when prepping for the interviews (Read: What Will a College Coach Ask Me...). Two big ones that may very well come out during this visit -- (1) Can you see yourself playing here? and (2) When can you commit?
While you won't be doing any on-field work, just keep in mind that you are still constantly being evaluated: your character, your responses, etc. A visit like this does not guarantee a spot on the roster. You've been identified as someone they'd potentially like to sign, and now they want to see if your personality fits the mold of person that they are looking for - they want an asset for the team, the school, and the community.
Make sure to get to know the school beyond that of the team and sports facilities as well. Remember, outside of baseball you'll be here for four years -- you need to know that you'll be happy and comfortable! We remind student-athletes that they should look to have an admissions tour as well to supplement this official visit. The admissions representatives will take note as you 'demonstrate interest' -- you'll be able to more fully immerse yourself into the day-to-day happenings of a student, and you'll be able to visit other areas of campus that you may not see with the coach/team. Other than a field, there will be two main places where you spend A LOT of your time -- the dining hall and the dorms -- be sure to not miss those.
With any questions on this, as well as anything else pertaining to athletic recruiting, just let us know by sending an email to firstname.lastname@example.org!