besides; in addition; furthermore~
その上 means 'besides; in addition; furthermore' and adds an additional point to what was just said.
その上 is used when you want to add another fact or reason to what you have already said. It is similar to 'on top of that' in English. The additional point often strengthens the argument or provides extra information. It is common in both speaking and writing, but it sounds a bit more formal than casual connectives like それに. When used in speech, it is often preceded by a sentence ending in だ/です or a plain past form. You can start a new sentence with その上, or connect two clauses with it after a period or comma.