Nelson’s chain of boats represents systemic power in which people living “within the machine” are literally or metaphorically chained together, preventing others from leaving even if they want to. Some humans are just “plugged in” to the illusions & constructs of society and cannot comprehend the ability to think for themselves. What’s more, they cling to their false reality with such vigor, they will vehemently oppose anyone who seeks to change or leave the enclosure. The Straw Hats, then, represent radical thinkers challenging the presumptions of society without regard for civility, and society is quick to try to capture them and prevent them changing the status quo in any way that might make the average person uncomfortable, even if only for a moment.
The chains on the boats create a trap but make sailing the ship difficult, completely restricting their individual freedom of movement. The attachment of the chains removes any semblance of creativity or exploration from the different crews, keeping this homogenous group moving in a uniform line of conformity with a singular goal demanded of them by their leader. Luffy & crew, seeking their own personal goals, must battle the much larger force to discover any chance of success. We see this in our own lives whenever we try to alter cultural norms and are confronted by those who adhere to, and often benefit from, those cultural norms. The leaders of real society have convinced millions that upholding their prescribed course of actions will truly lead them to happiness, much like the Commodore is using his fleet of crews to pursue his own personal goal.
Last note, Grandpa Rio is able to finally find the energy to rise up, save Apis, & fly because of pure, unadulterated, love. Love can be a wildly powerful motivator for anyone, bringing us the confidence and focus to achieve possibilities that would otherwise be genuinely impossible. Humans, and, apparently, dragons, are primarily social creatures, and maintaining the safety of our peers & companions can trigger strengths & abilities we have never harnessed before. For many, it is easier to do something for a loved one than it is to do something for one’s self, shown here by Grandpa Rio using his energy to try to destroy the Marines to save his friends, rather than use it to find the Lost Island.