To be infirm is to be physically weak. If your great grandmother can't get around without a walker or a wheelchair, you might describe her as infirm.
The adjective infirm is most often used to talk about elderly people whose bodies don't work as well as they used to — there's an implication of old age in the word. You could also describe an infirm person as decrepit or feeble. Sometimes a person's morals or character are described as infirm, which means you can't count on them to do the right thing. The Latin root, infirmus means "weak or feeble," or alternately, "superstitious or inconstant."