What is "Additional Summon"?

There will be a time where you will come across cards with the text.

"you can Normal Summon and/or Set #_____ monster(s) in addition to your Normal Summon or Set."

You can only have this effect once per turn even if you have multiple copies of monsters with this effects.

Ex.

Player 1 Normal Summons Verz Castor
Verz Castor the moment it hits the field grants the opponent another summon during the turn.
Player 1 Normal Summons another Verz Castor
Verz Castor #2 will not allow the player to summon another monster even if its conditions are met.

The "Hunder" is an exception to this ruling.