It's the approximate time the transaction has been added to a block, but since any node can assemble the transactions into a block at any time, it is not quite reliable.

In the replies above, you can see the "block time" the time when the transaction was added to the block (in seconds since epoch I think). However, actually converting it to a human-readable value is a different story, because of time zones, daylight-saving times, leap-seconds, and so on.