As others pointed out, "innocent" often means "not guilty," both in serious or joking situations. So "I'm innocent" means "I didn't do it," or "I'm not guilty."
Innocent is also commonly used to mean naive or pure, especially related to children.
The baby has such an innocent smile. He's adorable!
It can be used to describe adults or even teens too, but then it comes with some judgment attached that they are a bit naive or even childlike/immature.
"What does beer taste like?"
"Wow, you're so innocent!"
Lastly, it can also be used to describe someone's motives, and it means "pure" or "with good intentions."
1. I made an innocent offer to help Mary with her homework, but she thought I was flirting.
2. The city's new budget proposal was innocent enough, but it had unintended consequences for our community.