The OS has some “free” map data, but what am I allowed to use it for?
The full license text is available at the OS website. Looks relatively simple - you can use it commercially if you want, but you must attribute them.
You're asking a question that has legal implications. If you really need to know what you can or can not do with the OS data, you need to consult a lawyer. Either that or make your own interpretation, and assume the risk that it is incorrect.
These terms have been aligned to be interoperable with any Creative Commons Attribution 3.0 Licence. This means that you may mix the information with Creative Commons licensed content to create a derivative work that can be distributed under any Creative Commons Attribution 3.0 Licence.
If you want to use outside of that licence, then speak to a lawyer, or read over the licence yourself