Did u know what the gemimi mean in yugi or not !!! If not i will explain it. They are. Normal moaters in field and grave. But u can (RESUMMON then instead of do your normal summon) then he ill get his effect
So when u summon him he is a normal monster with no effect next turn u can re summon him and lose ur normal summon but the gemini get his effect
Lord 2022 is correct, but in simpler terms this card works like this.
Turn 1: normal summon him, at this point he is treated like a non-effect monster.
Turn 2: you can activate his effect and "resummon" him as an effect monster to gain the 2100 Atk.
*Keep in mind that "resummoning" takes place of your regular summon that turn.