How does incoming route advertisements works in BGP?

I am learning BGP on my own and I think I understand the concept of outgoing route advertisements...basically an AS, advertises to the world (hence the outgoing part) the prefixes it has in it's network...this outward advertisement leads to incoming traffic being routed to that particular AS.

I have now come across the concept of incoming route advertisements, and I am having a hard time picturing what it means. Basically the context I saw the term is the following:

"The order of preference for incoming route advertisements is Customer, Peers and then Providers"

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I am not sure I understand the above. :/