Although Groot sacrificed himself in Guardians of the Galaxy to save Peter Quill, Gamora, Drax and Rocket, he left behind a twig, which grew into the Groot that we saw run around last year in Guardians of the Galaxy Vol. 2. Many have believed that Baby Groot is simply the original Groot resurrected, like a phoenix rising from the ashes, but director James Gunn later revealed that wasn't the case, and today he stated matter-of-factly (during a debate over if you would save Groot or a Porg in a Sophie's Choice-like scenario, for full context) that the current Groot is actually Groot Jr.
This isn't the first time we've heard that Baby Groot isn't the same individual as OG Groot. In the lead-up to Guardians of the Galaxy Vol. 2, when asked if Baby Groot is the same Groot or if he's the son of Groot, James Gunn simply responded, "It's complicated." Shortly after Guardians of the Galaxy Vol. 2's release, Gunn clarified that Baby Groot is an entirely different character, as he wanted Groot's death in the first Guardians movie to mean something. Even after that, one could have worked on the assumption that Baby Groot is just a juvenile clone of OG Groot, but according to Gunn, we should be looking at the newest walking and talking tree as an offspring. Which makes sense, given that Baby Groot didn't have any memories of what happened in the first movie.
Baby Groot certainly shared some similarities with the original Groot, from his vocabulary being limited to "I" and "am" and "Groot" (exclusively in that order) to his occasional inability to follow simple instructions. However, it's 100% clear that we should not be looking at him as the same character. The Groot we enjoyed in Guardians of the Galaxy died when he used his body to form a protective shield around his friends so they could survive the Dark Aster's crash. He left behind a piece of himself, but what grew from that twig is an entirely new character who we've grown to appreciate.
For the majority of Guardians of the Galaxy Vol. 2, the new Groot was causing trouble and being all kinds of cute in his small, infant form, but during one of the mid-credits scenes, which took place years after the main story, we saw him as a "teenager/adolescent." It's hard to say when the new Groot will finally reach the same size as his predecessor, but now that's he going through whatever the sentient tree version of adolescence is, that's going to be especially fun for his teammates to deal with during their travels.