Smart contract transaction on Bitcoin network will be enabled.
smart contracts in the sense that you may think (like the etherum) will never be added to bitcoin but bitcoin has always had "smart contracts" from day one but only so much so that a currency needs.
taproot will add more capabilities.
Like all these are the results of the successful upgrade. What will change and what will be better.
you can read the topic here:
https://bitcointalk.org/index.php?topic=5140134.0or bips 341 342 and 343