Neji's seal is one of those world-building details that initially seems minor but gets heavy later on. It's called the 'Caged Bird Seal' and it's a Hyūga clan thing, specifically for the Branch Family members. Basically, the Main House puts this curse mark on Branch House kids to control them and protect the Byakugan bloodline. If a Main House member activates it, it causes excruciating pain, and if the Branch member dies, the seal destroys the Byakgan so enemies can't steal it. Neji's dad was in the Branch House, so Neji got it at a young age.
What makes it sting is the philosophy behind it. It's not just a power limiter; it's a symbol of predestination and oppression within the clan. Neji's whole 'destiny' speech to Hinata early on is directly tied to him being marked by this seal. It's a physical representation of the fatalism he struggles with for so long. The origin is purely a cruel, man-made system to maintain hierarchy, which makes his eventual rise and the partial reform of the clan more impactful. I always found it a more grounded kind of tragedy compared to some of the bigger, magical curses in the series.
Its initial purpose was pragmatic and brutal, a plot device to explore themes of freedom versus fate.