It means "to prepare," to do the little things that are necessary for later success.
(Many) plants grow from seeds. Typically, after plowing the soil to soften it, the farmer sows the seeds. "Sow" is pronounced exactly the same as the words "so" or "sew." To "sow" seeds means to scatter them on the soil. You sow the seeds now, so that they will grow into plants later.
"To sow the seeds" is often used figuratively to mean "to prepare." It refers to early preparation. It means to do something that seems small, like drop a tiny seed on the ground, that may grow into something enormous later.