Offred has a tattoo on her ankle of four numbers and an eye. This is to ensure that, like all the other handmaids in Gilead, she will be readily identifiable. The tattoo is a symbol of the repressive, controlling nature of the authoritarian society in which Offred lives. This is a society where women and their bodies are considered nothing more than the property of men, theirs to do with as they please. Offred's tattoo acts as a constant reminder that her body is not her own; it's public property that exists purely as a means for procreation.
The numbers on the tattoo show to what extent Offred and all the other women in society have been stripped of their basic humanity; they are no longer humans, just numbers. And the eye represents the all-seeing surveillance of this theocratic state, where everyone is under the ever-vigilant gaze of the authorities.
During the Holocaust, Jewish prisoners in concentration camps were marked with numbered tattoos by which they were identified. By adopting a similar method of public branding and control, the authorities of Gilead are openly displaying the hatred—in this case, hatred of women—on which their brutal regime is based.