Can someone explain the title to me? Is it short for pikachu API?
Probably just that Pikachu often says “pika pika”, and sometimes variations like “pika pi”
No, just one of the handful of things that Pikachu can say.
Pretty sure Pikachu’s pikapi is him calling Ash’s name. Closest thing Pikachu can say to Ash’s Japanese name, Satoshi.
I didn’t know about this until now, that’s so cool! Apparently there’s a whole list of words he uses somewhat consistently: Bulbapedia
Ash's Pikachu - Bulbapedia, the community-driven Pokémon encyclopedia

That’s what Pikachu usually calls Ash.
It’s Pikachu-speak for Satoshi, aka the Japanese name for Ash.

It’s Pikachu-speak for Satoshi, the Japanese name for Ash.

Pi(Sa)ka(to)-pi(shi)

Yeah. In the image, Ash is just RESTing.