A Flip Summon is a way to Summon Monsters. You can only perform it when the game state is open during your Main Phase. You simply flip a face-down Monster manually into face-up Attack Position. A Monster is not considered to be on the Field during the Flip Summon. A Flip Summon is simultaneously regarded as both a Summon and a Battle position change.
Not a Flip Summon
A face-down Monster flipped into face-up Attack Position by a Card Effect such as "Book of Taiyou" is not considered a Flip Summon.
Not your Normal Summon/Set
A Flip Summon does not take up your Normal Summon/Set for the turn.
You cannot Flip Summon a Monster at the same turn it was Summoned or Set.