Q: Texting via MacBook Air

Yes. Enable Text Message Forwarding on your iPhone. To learn how to do that read Use Continuity to connect your iPhone, iPad, iPod touch, and Mac - Apple Support and scroll down to "Set up SMS and MMS with Continuity".

You can send an iMessage to someone that has an iPhone that has iMessage turned on: Mac Basics: Messages helps you chat instantly - Apple Support

If you also have an iPhone you can use your Mac to send a text message from your Mac. The message is routed from your Mac through the iPhone using Continuity: Use Continuity to connect your iPhone, iPad, iPod touch, and Mac - Apple Support

In addition to the responses above, you can also use email to send a text message from a Mac if you know the recipient's cellular carrier.

For example, to send a text message to someone using AT&T with the phone number (987) 654-3210 in the U.S., compose and send an email to 9876543210@txt.att.net

lets say they don't have a iPhone and u want to send a text, you can always send messages thru Skype!

Have them download Skype on their phone and u can download it on your mac and than, message away