Okay kiddo, so let me explain "la onda" to you.
Imagine you're at the beach and you see waves coming in to shore. They go up and down, up and down, right? That's kind of like "la onda".
It's a word in Spanish that can mean a lot of things, but usually it's used to talk about a certain vibe or feeling. Sometimes people might say "qué onda?" which means "what's up?" or "how are you feeling?"
So when people talk about "la onda" they might be talking about the general atmosphere or feeling of a place, like a party or a concert. If everyone is really happy and having a good time, you could say "la onda está muy buena" (the vibe is really good). Or if people are feeling kind of sad or down, you might say "la onda está pesada" (the vibe is heavy).
Overall, "la onda" is just a way of describing the feeling or energy of a situation or a group of people. Does that make sense to you?