This type of geocache is known as a "lamp skirt" or "LPC" standing for "Lamp Post Cache". This is considered one of the easiest and most common types of hides. The secret is that the squarish metal box on top of the concrete base of the lamp post can usually be lifted up the pole, revealing a handy little hiding place underneath.
Sometimes there are tricky variations:
-- A magnetic container on the underside of the metal "skirt"
-- A camouflaged magnetic container on the pole itself, hidden "in plain sight"