1. make something simpler and easier for people to understand
dumb down a speech so that the audience can better understand it
The expression to 'dumb something down' means to simplify something or explain something in a way that is easier to understand or more accessible to others. The word 'dumb' however has a very negative connotation and is usually used as an insult. The impression here is that you are simplifying something or explaining something so that even someone who is not very intelligent can understand.