"Smashing" is a fancy-sounding word that means "great" or "wonderful". You can use it to talk about things that are very successful, very fun, or very beautiful:

A: How did it go?

B: It was a smashing success!

That dress is absolutely smashing!

"Smashing" is a little uncommon. In most situations, we use other words like "wonderful", "great", or "beautiful" instead.