The tree represents those who meditate on God's word so as to know it, and to know God, through his word.
The wicked do not do this. As a result, they have no depth. No substance. Nothing to hold them firmly and securely in place.
How are you connected to God? Are you deeply rooted in his word?
A couple of years ago I recorded a short audio reflection on this psalm. Below, I've linked to a post that references this audio and also includes some information about why it is helpful and useful for us to meditate on God's word.
Psalm 1 Audio