It is a fishnet pattern keffiyeh (Palestinian headdress), as she seems to be wearing it around a lot to appear as the most pro-Palestinian candidate.

From NPR

Some say patterns on the keffiyeh symbolize different aspects of Palestinian life: the bold black stripes on the edges symbolize the historical trade routes that used to go through Palestine; the fishnet-like design represents the Palestinians’ ties to the Mediterranean Sea; and the curvy lines resemble olive trees, a major point of pride for Palestinians.

Though none of these claims can be backed up by historical evidence, over the past 10 years they’ve become embraced by Palestinians in the diaspora to be the meaning behind their keffiyeh’s patterns.