What's the meaning of "in addition to" in this context

"Addition" is the noun form of "add", as in 2+2=4, or in a non-mathematical sense, to combine two or more things in some way.

So "in addition to" means to add one thing to another, that is, to have or use both. When we say "A in addition to B" we normally mean that you already have or plan to get B, and someone is saying that you will or should also get A. In the example you give, the writer is saying that it is expected or assumed that you will use a database, and he is saying you should also use "Solar".

Likewise one could say, "You should wear socks in addition to shoes", meaning we know you plan to wear shoes, but you should also wear socks. Etc.