Not many people know that tyres are comprised of many layers of rubber, fabric and even steel. Please see the below diagram of a tyre cross-section.
Due to these different components, your new tyres require a break-in period to ensure that they deliver their normal ride quality and maximum performance. As tyres are cured, a release lubricant is applied to prevent them from sticking in their mold. Some of the lubricants stays on the surface of your tyres, reducing traction until it is worn away. Five hundred miles of easy acceleration, cornering and braking will allow the mold release lubricant
to wear off, allowing the other tyre components to begin working together. It is also important to note that your old tyres probably had very little tread depth remaining when you felt it was time to replace them. Don't be surprised if your new tyres are a little slower to respond (even if you use the exact same tyre as before). Their new, full depth brings with it a little more tread squirm until they wear down.