Fiera literally translates to "wild beast". It's a powerful word used to describe a fierce, untamed animal.
In "Ven Conmigo", Daddy Yankee uses it as a metaphor for raw, untamed energy on the dance floor. When he says "Saca saca la fiera" (Bring out the beast), he's hyping up the crowd, telling them to let go of their inhibitions and unleash their inner party animal. It's a perfect example of the high-energy slang that makes reggaeton so exciting.
This song is a classic party anthem with a simple story. Daddy Yankee spots a girl he likes in the club, falls for her eyes, and sees her leaving. Instead of letting her go, he makes his move. The whole song is his attempt to convince her to leave with him.
He pleads, "Ay mami ven conmigo / Es muy temprano para regresar a la disco" (Hey baby, come with me / It's too early to go back to the club). He promises her an amazing time, telling her to put her wallet away because "la casa invita" (the house is treating). He even gets a little wild with his promises, saying, "Tengo la nave afuera / Vamos a mi planeta" (I have the ship outside / Let's go to my planet). It's all about dancing, having fun, and convincing this one person that the party is wherever he is.