What does Genesis 38:19 mean?

Judah, on his way to celebrate sheep-shearing season, has stopped to buy the services of a roadside prostitute. What he did not realize is that this was his own widowed daughter-in-law, Tamar (Genesis 38:13–18). Tamar was not really a prostitute, but a desperate woman in disguise. She had been widowed by Judah's two oldest sons and then abandoned by him when the strong custom of the day—and his previous agreement—demanded she marry his third son (Genesis 38:11). When that did not happen (Genesis 38:14), Tamar had taken matters into her own hands.

After having sex with Judah in her guise as a prostitute, Tamar took his signet, cord, and staff as security against a future payment. She changes back into her widow's mourning clothes and goes home. Her plan, at this point, was likely to use Judah's signet and staff as evidence to shame him into following through on his promise. Realizing she is pregnant, however, would lead her to understand that her best option is to keep the items and wait (Genesis 38:24–25).