means that something is completely covered with; full of something.
It is mostly used for undesireable things.
It can also be used in a more figurative way.
分からないことだらけだな。 I'm full of things I don't understand
スパイダーだらけだ。 the area is completely covered in spiders.
{相変|あいか}わらず傷だらけですが… As usual, it's covered in scratches, but...