It's a way to express that you are not joking or being playful, and that you mean what you are saying.
Here are some examples of how you might use it:
* "Soy serio, no estoy jugando." (I am serious, I am not playing.)
* "Te lo digo en serio, tienes que tener cuidado." (I'm telling you seriously, you have to be careful.)
* "Soy serio, necesito hablar contigo." (I'm serious, I need to talk to you.)
It's important to note that the tone and context can affect the meaning of the phrase.