What does Genesis 38:15 mean?

Judah told his widowed daughter-in-law Tamar she would be married to his son, Shelah, when the boy grew up (Genesis 38:11). Tamar has realized that's not going to happen—Judah's reassurance was a lie (Genesis 38:14). In that culture, a childless widow (Genesis 38:7–10) had no realistic prospects for marriage and no family to care for her into old age. Facing a dire future, Tamar is working a scheme to force Judah to take care of her.

She has dressed herself as a prostitute, wearing a veil, and is waiting for Judah to pass by on the road (Genesis 38:13). Her plan works. She catches his eye, and Judah assumes she is, in fact, a prostitute. In part because of the veil, he never realizes she is his daughter-in-law. In the following verses, he will hire her services. Tamar will use this encounter to her advantage.