Among other things, it usually means that the file type has wide interoperability (which makes it more likely you can open it in the far future) and comes in a

Among other things, it usually means that the file type has wide interoperability (which makes it more likely you can open it in the far future) and comes in a format resistant to damage, so if bits are changed or removed, you can still recover the rest of the document (usually this means avoiding compressed formats). As to how well-suited PDF/A is for these aspects, I'm not experienced enough to say.