To spell out something (or spell something out) is to explain it in a very clear and detailed way.
Examples of use:
1. I don’t understand what you are talking about. Can you spell it out for me?
2. I’m sure I don’t have to spell out how important it is to practise your English as often as possible.
infinitive – spell out
present simple – spell out and spells out
-ing form – spelling out
past simple – spelt out
past participle – spelt out
Image by kevindooley