It's about Jessica getting pregnant at a young age(don't really remember hahahaha)? So he parents got mad so they give her to her grandma. 

Taeyeon is a friend of sica's grandma.

Taeyeon help sica until she give birth and leave her baby in taeyeons care.

Years pass by sica is a CEO and Taeyeon is a doctor. They met again when sicas daughter saw her and cried.