A conjunction is when two things appear close together in the sky as seen from Earth. The planets, the Moon and the Sun seem to wander around compared to the stars, which appear fixed. As the wondering objects travel around they sometimes appear to get close to one another or to other astronomical objects such as star clusters. There is no strict definition of how close the two objects have to be to for them to be considered as being in conjunction. Some people say they have to be within a degree of each other but really you can just make it up. Even quite widely spaced conjunctions can look really cool.
P.S. There is a more technical definition of a conjunction which is that the two objects have to have either the same right ascension or the same ecliptic longitude, but we’ll talk about these things another day.