show processlist
One of the possible status of a running query is "Sending data"
However, the meaning of the status is wider since the process is fetching and filtering data.
This means that MySQL has some data stored on the disk (or in memory) which is yet to be read and sent over. It may be the table itself, an index, a temporary table, a sorted output etc.
If you have a 1M records table (without an index) of which you need only one record, MySQL will still output the status as "sending data" while scanning the table, despite the fact it has not sent anything yet.