What does "pasa" solely mean?

What does “pasa” solely mean?

It can mean several things (including “pass”, by the way, “¡Pasa la sal!”)
In this case “pasar” means “to happen” [think of “it came to pass”]. “Pasa” is the third person singular of the present tense, meaning “it happens”.
“¿Qué te pasa?”, literally “¿What’s happening to you?”, is used here in the sense of “What’s the matter with you?”

A silly example so you can remember the expression more easily. A couple of decades ago there was a stupid song in Spain, whose lyrics consisted mostly of senseless sounds, completed with a sentence in both Spanish and broken English, thrown in just for the heck of it. The song says:
“¿Sabes lo que pasa cuando dices que me quieres? You know what happens when you tell me you love me?”, English pronunciation is just horrible.
Link to the song, notice that the description says “canción cutre”, meaning something like “crappy song”

When I was in Mexico I heard “pasa” used quite a bit, with the meaning “go ahead.” In a typical crowded tienda, the owner would cry out, “Pasa, pasa!”, meaning “Don’t be so hesitant, big American guy, there’s enough room for you!”

In reading, I’ve never encountered this usage.

It’s a very usual meaning, of course you’ll find it in reading.
The primary meaning of “pasar” is to “go by”. In the sense you mention it just means “do come in”, it doesn’t need to have any kind of implications about being “gringo” or hesitant.
Every time you’d say “come in” in English, it would be “pasa” in Spanish.

Thank you for you help =)

Thank you for your help =)

I didn’t mean to suggest it had anything to do with my nationality. I’m 1 meter 90 and I’m a giant in southern Mexico and Central America where the average height is around 1 meter 65. My hesitation was a matter of my being tall and clumsy. In my experience, Latin Americans have always been very courteous. Thanks for helping to clarify this.

I’m glad if it helped.