It’s no join since the relationship will only be evaluated when needed. A join (in a SQL database) on the other hand will resolve relationships and return them as if they were a single table (you “join two tables into one”).
You can read more about DBRef here:
There are two possible solutions for resolving references. One is to do it manually, as you have almost described. Just save a document’s _id in another document’s other_id, then write your own function to resolve the relationship. The other solution is to use DBRefs as described on the manual page above, which will make MongoDB resolve the relationship client-side on demand. Which solution you choose does not matter so much because both methods will resolve the relationship client-side (note that a SQL database resolves joins on the server-side).