True to its title, this spinoff tells the story of Nagi's journey in Blue Lock - though with how significant of a role Reo plays in Nagi's journey, Reo can be considered akin to being the second protagonist of the story. The first chapter starts off right after Nagi and Reo make their promise to play football together and shows how the two became a powerful duo pre-Blue Lock, continuing on to how they got invited into Blue Lock and how they passed the selections. If you like Nagi and Reo, you'd definitely enjoy reading this spinoff as it adds more depth to their relationship and gives a better understanding of their thoughts and ego/motivations. Some side characters that aren't as relevant in the main series, such as Zantetsu, are also given some spotlight.
You're probably wondering if the scenes/matches in the main series Nagi is in are repeated or skipped over, and the answer (from what we're shown so far, anyways) is the former - except of course, we get to see how things plays out in the eyes of Nagi instead of Isagi. As such, there is repeated material from the main series, but the pacing seems to be much quicker in comparison. Some people may find these parts of the spinoff repetitive, but I for one appreciate it as it highlights Nagi's character as a foil to Isagi and adds layers to his character that the main series is lacking; Nagi is the opposite of expressive, so he is easily mischaracterized and thus being able to see his thoughts offers the readers new insight into his character and how he views playing the sport/other characters.
Overall, this spinoff is arguably as hype and enjoyable to read as the main series - especially thanks to the amazing art, which matches the main series' art in terms of intensity and flair. Recommended to anyone who likes Blue Lock and wishes to learn more about Nagi and his fellow Blue Lock teammates/rivals.