As with the other three aces, (Wands, Cups, and Swords), the symbol is a gift from above (the hand emerging from a cloud). This gift is given in a garden sheltered from the wilderness. The magic inherent in the natural world and life on Earth gives us the raw materials to create our civilisation, a safe and comfortable environment for ourselves.
The mountains through the gateway symbolize greater knowledge available to us when we are ready to leave the garden in search of deeper truth.
Reversed: wealth corrupts, and material gifts are abused, bringing selfishness, extreme competition, mistrust, or over-dependence on security and comforts. Or, on a deeper level, the protection of the garden is lost, and we are forced to stand on our own feet, or to leave the familiar behind and travel through the gate to the mountains of wisdom.