Psalm 81:9, "There shall be no strange god in you, neither shall you worship any foreign god," is a straightforward command emphasizing exclusive devotion to the Lord. Let's break it down:

"There shall be no strange god in you": This part of the verse focuses on the internal aspect of faith. The phrase "in you" can be interpreted in a few ways:
In your heart/mind: It means you shouldn't harbor beliefs in or affections for other gods. It's about rejecting polytheism and maintaining monotheistic devotion.
In your land/community: In the context of the Israelites, it means they shouldn't allow the worship of other gods to take root within their nation.

"Neither shall you worship any foreign god": This part focuses on the external aspect of faith. It forbids any outward acts of worship toward other deities. This includes sacrifices, prayers, rituals, or any other form of veneration. "Foreign" refers to gods not belonging to the covenant relationship between God and His people.

In essence, the verse is a clear and unequivocal command to:

Be exclusively devoted to the Lord.
Reject any allegiance to other gods, both in thought and in action.
Maintain the purity of worship.
Avoid syncretism (mixing of religious beliefs) with the practices of other cultures.

The context of Psalm 81 is a reminder of God's deliverance of Israel from Egypt and a call to obedience to His covenant. The command against strange gods is a fundamental element of that covenant and a condition for enjoying God's continued blessing.

In a broader sense, this verse can be interpreted as a call to avoid idolatry in all its forms, including the worship of material possessions, power, or anything else that takes precedence over God in one's life.