What does FTS_ file mean after converting from MyISAM to InnoDB? I have convert some tables in database from MyISAM to InnoDB. After that, I see some modification on file system.

The size of database is grown up, from 2.8GB to 4.8GB and there are many file with prefix FTS_* like

FTS_000000000000005c_00000000000000f3_INDEX_1.ibd
FTS_000000000000005c_00000000000000f3_INDEX_2.ibd
FTS_000000000000005c_00000000000000f3_INDEX_3.ibd
FTS_000000000000005c_00000000000000f3_INDEX_4.ibd
FTS_000000000000005c_00000000000000f3_INDEX_5.ibd
FTS_000000000000005c_00000000000000f3_INDEX_6.ibd
FTS_000000000000005c_BEING_DELETED_CACHE.ibd
FTS_000000000000005c_BEING_DELETED.ibd
FTS_000000000000005c_CONFIG.ibd
FTS_000000000000005c_DELETED_CACHE.ibd
FTS_000000000000005c_DELETED.ibd
FTS_0000000000000075_0000000000000118_INDEX_1.ibd
FTS_0000000000000075_0000000000000118_INDEX_2.ibd
FTS_0000000000000075_0000000000000118_INDEX_3.ibd
FTS_0000000000000075_0000000000000118_INDEX_4.ibd
FTS_0000000000000075_0000000000000118_INDEX_5.ibd
FTS_0000000000000075_0000000000000118_INDEX_6.ibd
FTS_0000000000000075_BEING_DELETED_CACHE.ibd
FTS_0000000000000075_BEING_DELETED.ibd
FTS_0000000000000075_CONFIG.ibd
FTS_0000000000000075_DELETED_CACHE.ibd
FTS_0000000000000075_DELETED.ibd

I am using MariaDB and turning innodb_file_per_table on.
These files are related to the new FULLTEXT index support in InnoDB (starting with MySQL 5.6). Removing them is not a good idea for sure.

Unlike with MyISAM, where FULLTEXT indexes are stored in the tables .MYI file together with all other indexes fulltext indexes in InnoDB are implemented using several internal helper tables which show up as separate .idb table space files in innodb_file_per_table mode:

The "...INDEX_1.ibd" to "...INDEX_6.ibd" table files contain the actual fulltext index, split up into 6 partitions. The "...CONFIG.idb" stores meta information about the fulltext index and the "...DELETED..." files are needed for housekeeping of deleted data. 

For more details see

http://dev.mysql.com/doc/refman/5.6/en/innodb-fulltext-index.html#idm140684547617952