here's how i understand it. cancer cells that are metastasizing need to detach, migrate, and stick to basement membrane in order for those cells to eventually break through the basement membrane (to get to other areas). laminins and integrins are proteins that allow the cancer cells to attach to the basement membrane (in the same way that integrins allow rolling neutrophils to stick to endothelial basement membrane wherever they need to migrate).
as a side note, cancer cells that metastasize often manufacture matrix metalloproteinases (MMPs) that allow them to break through the basement membrane.