It an expression that means “I agree with what you just said”

It's a bastardisation of the Latin idem dito

Ditto (Japanese: メタモン Metamon) is a Normal-type Pokémon introduced in Generation I.

In its natural state, Ditto is a light-purple or pink blob-like Pokémon with vestigial facial features. It is often referred to as amorphous, but has a relatively consistent appearance in official artwork, including two small nubs on its "head", a few soft lumps at its base, and two pseudopod-like protrusions in place of arms. The face consists of beady eyes and a simple mouth; almost always pulled into a smile.

It is capable of transforming into an exact replica of any physical object or living creature, including its form and abilities. Each Ditto has its own strengths and weaknesses when it comes to transforming; being unable to remain transformed while laughing and getting details of its transformation wrong if based on memory being apparently universal.

It means "I feel the same way you do and would have said it myself, so I acknowledge and applaud that you said it first".