She explains that when she was a child a witch cast a spell on her, causing her to transform into an ogre when the sun goes down, and that the curse can only be broken by true love's first kiss.

Is Fiona an ogre because her dad is a frog?

In Shrek, Fiona changes from a human during the day to an ogre at night and the events of the first movie make the change permanent. However during the second movie we learn that her father the king was actually the frog prince, and was turned human by the fairy godmother so he could marry a human.

Why is Fiona's dad a frog?

Prince Harold was born as a frog and lived in the kingdom Of Far Far Away. He sought the help of Fairy Godmother, who gave him a magical potion that made him human so that he could court Princess Lillian. Their first date was by a lily pond and their first kiss permanently made him human.