Can I use an iPad as a second screen for my MacBook Air?

Perhaps you mean Sidecar? Apple describes how to achieve this in the support document Use an iPad as a second display for a Mac in which Sidecar is described: With Sidecar, you can use your iPad as a display that extends or mirrors your Mac desktop.

The system requirements are

iPadOS 13 or later

• iPad Pro (all models)
• iPad (6th generation) or later
• iPad mini (5th generation) or later
• iPad Air (3rd generation) or late

macOS Catalina or later

• MacBook Pro introduced in 2016 or later
• MacBook introduced in 2016 or later
• MacBook Air introduced in 2018 or later
• iMac introduced in 2017 or later, or iMac (Retina 5K, 27-inch, Late 2015)
• iMac Pro
• Mac mini introduced in 2018 or later
• Mac Pro introduced in 2019
• Mac Studio 

An iPad Pro can be used with a compatible Mac to extend the Mac display to the iPad using the Catalina/iPadOS Sidecar feature.

Use iPad as a second display for your Mac – Apple Support (UK)

For older models of Mac and iPad that do not support Apple Sidecar, or if you would prefer to use something different, perhaps consider an alternative third-party product - such as Duet. Diet can be used to extend a Windows PC or Mac to an iPad:

https://www.duetdisplay.com/