What's the deal with rain checks?

I asked this girl out to lunch and she said she already had plans with friends for the entire afternoon. Then she said: sorry! rain check for next week maybe?

Is it worth it to ask her one more time? I can't help but get the feeling she is trying to say thanks, but no thanks.
Updates
+1 y
Forgot to ask this. If we do set another date, should I continue to casually text her until then or wait. We have texted back and forth a little up to this point.
What's the deal with rain checks?
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