Trees Problems

I just addded some trees in my unity 3d game and i dont understand why i can walk thru them
Just like a ghost…Any ideas?

When making almost any 3D game, objects need Colliders to have collision detection.

For the tree, just drag the prefab into the scene view, select it, and go Components > Physics > Capsule collider. Make sure it is the shape of your tree trunk, and fits well, then locate the tree in the Project Window. Now drag the Tree you’ve just edited from the heirachy view onto the original Tree Prefab in the project view.

Voila! You’ve just made a prefab. Now every tree of that kind you place will have a collider on it.