"Cargo cult" is a type of cult which appeared in the Melanesia, in which the natives would build their copy of an airplane (no motor, 'cause they didn't have the knowledge to build one -- or even knew what went inside the airplane) in the hopes they would get the same results as a real airplane.
In IT, a "cargo cult" is the expectation that if you use the same tools as some big players, you'd end up getting the same results.
One example: Netflix runs a large fleet of microservices daily; they use Spring Cloud; if we use Spring Cloud, we can also run a large fleet of microservices.
Although it may sound correct in a first glance, things are not like that. There is much more to the Netflix fleet than just Spring Cloud.
Sometimes, cargo cult can appear in a form of "fanaticism" about celebrities: Fowler said such and such pattern works this way and that's exactly what we should do. Just because Fowler is well know software engineer and architect and do have some very clever ideas, picking them and running exactly the way he described may do more harm than good -- basically, 'cause you'd end up applying a design pattern without worrying about solving your problem in the first place.
Another example: "ProductX is sponsored by BigCompany, so it's good". It may be, but the fact that BigCompany is being ProductX doesn't immediately makes ProductX good, or even if it fits your solution. And there is much more behind a product than just its development.