Join implies bring two things together, in database context, we are usually talking about two tables. Multiple-table join refers to joining three or more data tables together using one query or one action command. There is no upper limit in the number of tables a user can combine in one action command, theoretically infinite.
The actual operation is done by layering multiple join statements together. This means joining two tables together first, and then joining the third table to the finished product of table one and two, and then joining the fourth table to the combined product of the first three, and so on. The multiple join statements can be of different join types (inner, left, right, full, etc.) but in general inner and left joins are used more often.