While the unbelievers set up in their hearts the arrogance, the arrogance of ignorance, Allah sent down His tranquillity upon His Messenger and the believers and made the believers to adhere to the word of piety; for they were most worthy and deserving of it. Allah has full knowledge of everything.
Meaning:-
Arrogance: an insulting way of thinking or behaving that comes from believing that you are better, smarter, or more important than other people
Ignorance: lack of knowledge or information.
Tranquility: peacefulness, restfulness.
Adhere: believe in and follow the practices of.