Aloud means out loud, so that other people can hear. Not in your head.
It follows the verb, as in reading aloud, thinking aloud, saying it aloud. Reading aloud is the standard phrase for reading to somebody, and thinking aloud is a way of warning people that what you are saying is not yet a decision. Take care with allowed, which means permitted and sounds identical, and take care with loudly, which is about volume, since you can say something aloud very quietly indeed.
1adverbaloudIn a voice that can be heard by others.
“She read the letter aloud to the whole room.”
Middle English
aloud
English, today
aloud
From Middle English aloud, a loude (“aloud”), equivalent to a + loud or a + loude.
Etymology from Wiktionary, CC BY-SA