Simple Example: Lets say you have a Students table, and a Lockers table.
Each student can be assigned to a locker, so there is a LockerNumber column in the Student table. More than one student could potentially be in a single locker, but especially at the beginning of the school year, you may have some incoming students without lockers and some lockers that have no students assigned.
For the sake of this example, lets say you have 100 students, 70 of which have lockers. You have a total of 50 lockers, 40 of which have at least 1 student and 10 lockers have no student.
INNER JOIN is equivalent to "show me all students with lockers". Any students without lockers, or any lockers without students are missing. Returns 70 rows