These are moisture vaporators. These devices are used for collecting moisture from atmosphere that will be used as water.
From Star Wars databank,
Found on desert planets, moisture vaporators were stationary devices that could collected moisture from even the driest of climes. Though they were stationary, they included programming that offered them a binary language that could be used to communicate with droids. On the desert planet of Tatooine, edible mushrooms frequently grew on the vaporators. For moisture farmers to make a living, they would need fields of moisture vaporators, and then security systems to protect them for those who might steal their water.
These get mentioned by Luke's uncle when they purchase droids from jawas.
OWEN: I have no need for a protocol droid.
THREEPIO:(quickly) Sir -- not in an environment such as this -- that's why I've also been programmed for over thirty secondary functions that...
OWEN: What I really need is a droid that understands the binary language of moisture vaporators.
THREEPIO: Vaporators! Sir -- My first job was programming binary load lifter... very similar to your vaporators. You could say...
Also see the same question on SFF.