Can I sync my Macbook Air with my iMac

Depending on what you want to do, there are several ways to do it.

There's iCloud. If you use iCloud Photos on both devices, photos that you add on one device will automatically show up on the other. If you put files into iCloud Drive, they'll be visible to both devices, etc.

Introduction to iCloud - Apple Support

There's AirDrop.

Use AirDrop on your Mac - Apple Support

You can also copy files onto USB flash drives and move them between Macs that way … though I wouldn't rely on flash drives as your only means of long-term storage.

Desktop and Documents is designed for that need: Add your Desktop and Documents files to iCloud Drive - Apple Support

To merely copy files from one Mac to another you can use any number of methods including connecting them through your existing network, or by using an external USB flash drive, hard disk drive, etc.

Connect your Mac to shared computers and servers - Apple Support

Connect and use other storage devices with Mac - Apple Support