Freestanding things are unattached or separate from any other structure or support. A freestanding garage isn't connected to a house — it's a separate building.
Some houses, like row houses or townhouses, are connected to the house next door, sharing a common wall. Freestanding houses, on the other hand, are completely detached, with space between each. You can also use the adjective freestanding more figuratively, to describe something that isn't reliant or connected: "It's a freestanding literary club — it's completely separate from and independent of the school."