If you’re talking to someone and they’re always looking down, it could be a sign that they’re afraid, guilty, or submissive. Instead of looking at you, they’re looking at the floor. Depending on the context of what is happening, this could mean any of the mentioned aspects. If they’re facing left while looking down, this could indicate they’re talking to themselves. If they’re facing right, they could be thinking about something personal. Of course, these directions could be switched from person to person.
If the person you’re talking to is from another culture, it may be normal for them to look down. In some countries, eye contact is a sign of disrespect, and is avoided whenever possible.