What does it mean when someone says you have crabs?

It means that you have Pediculus pubis, a scientific term for such parasite that survives up to 24 hours in your body by sucking your blood.

You can contract these parasite through skin-to-skin contact with another person or sleeping in an infested bed or clothing, even towels. Usually, a common symptom is scratching your pubic area. These can be spotted generally in your pubic area, but can also find them in your armpits, eyelashes, beard/mustache, and sometimes in your hair. If you look closely you can see these crab-like parasites that may be whitish-gray or rusty in color.

I strongly suggest getting treatment. Premethrin or Lindlane (Do not use Lindlane if you're pregnant, breast-feeding or under 2 years old). Wash and dry your clothing in hot water (2 weeks in a tight plastic bag for 2 weeks if you're unable to wash them).