Prendiste comes from the verb prender, which means "to light" or "to turn on." While it can refer to physically lighting something, it's often used metaphorically.
In this song, the lyric "Prendiste la luz y lo hiciste brillar otra vez" (You turned on the light and made it shine again) uses prendiste to signify bringing brightness and joy back into the singer's life, making it a beautiful and evocative choice.
"Mi Felicidad" ("My Happiness") is a love song about a dream coming true.
The singer describes waiting a long time for someone, then finally meeting them and realizing they had already imagined this person their whole life. Falling in love happens instantly, almost without trying.
The chorus calls this person "the man of my dreams" and "my happiness," someone who is there to stay. It is a simple, joyful song about the moment a long wait for love finally pays off.