@sachin16 - Welcome to the community!

The Subject Line is the actual “Subject Line” of the email when your campaign is sent.  And the Preview Text is typically the content that follows the Subject Line in most modern email clients.  Typically, email clients show the first sentence of text it can find in your email, but with Klaviyo, you can pre-define what that text will show.  

Here’s an example from someone using Gmail on the web:

Just note, the preview text’s length varies between email clients.  Some email clients show more, and some show less - and sometimes it depends on how wide the window is on Desktop. You can see the example above it cuts off after the first sentence.  It’s generally a good practice to put the most important text up front in the preview text as some text may be cut off at the end.

Example (Assuming 15% Off is the most important part):

• Good: 15% Off your next order with us! Check out our variety of colors from our warehouse today!
• Not as Good: Thousands of colors to choose from warehouse. Don’t forget, you’ll get 15% Off your next order!

Finally, if you don’t know what to use for Subject Line or Preview Text, you can always use Klaviyo’s AB Testing for Campaigns to try different things and let the results decide!

Here’s some documentation that might help:

• How to Insert Preview Text into an Email : Add Preview Text to a Campaign
• Guide to A/B Testing Campaign Emails

Hope this helps!