What is the meaning

That would be an ecumenical matter.

It means that one part is just as good as any other part. It originated in, or at least, was popularized in, a Wendy's TV commercial from the 1980s, and they're talking about chicken parts:

https://youtu.be/OTzLVIc-O5E

I've heard it used in reference to car parts, usually jokingly (the Wendy's commercial is mocking the idea that "parts is parts" too). There are usually a few different tiers for a particular car part: the cheapest "value" version and a more expensive "premium" version from the local auto parts store, and then an even more expensive factory replacement part from the dealership. Beyond that there may be even more expensive performance parts available, both from the factory and from aftermarket companies.

An example would be:

Auto parts store clerk: "We have one for $29.95 with a one-year warranty and a premium one for $44.95 with a lifetime warranty..."

Customer: "Give me the cheaper one. Parts is parts!" (customer laughs; clerk gives a courtesy laugh to the same "joke" that he hears forty-eleven times a day)