The Lord (God) acts as a shepherd to the speaker. He makes sure the speaker isn't lacking any necessities.
The Lord takes the speaker to peaceful and relaxing places, like green fields and calm waters.
He also tends to spiritual well-being, making sure that the speaker stays on the right path.
The speaker walks through the valley of the shadow of death, but he doesn't need to be afraid. The Lord will continue to protect him like a shepherd protects his sheep.
Even when his enemies are at hand, the speaker can enjoy a fine banquet thanks to the generosity of the Lord. He lives in abundance, even luxury, with rich oils and more wine than his cup can handle.
This happy state of affairs will continue for the rest of the speaker's life, and beyond. He doesn't ever plan to leave the protection of his host and shepherd.