Hive has a relational database on the master node it uses to maintain track of state. For example, when you CREATE TABLE FOO(foo string) LOCATION 'hdfs://tmp/';, this table schema is saved in the database.
If you have a partitioned table, the partitions are saved in the database(this allows the hive to utilize the lists of partitions without going to the file-system and finding them, etc). These kinds of things are 'metadata'.
When you drop an internal table, it drops the data, and it will also drop the metadata.
When you drop an external table, it only drops the metadata. That means the hive is ignorant of that data now. It does not encounter the data itself.
If you want more information regarding the same, refer to the following video: