I think this question is opinion-based, but not to an extent where it cannot be reasonably answered. Anyway, that's a decision someone else can make.
Glasslip is about teenage life, but without many of the cliches that are in many shows - characters are uncertain about many things - love, friendship, future, and in general growing up.
This is why not much happens during the show - the characters are nervous about the fact that they can now date within the group, that a new person has disrupted their social circle, that they will soon be separated as they move on to new schools and new areas.
These uncertainties are shown abstractly in character's interations:
Touko feels 'invisible' in her vision of watching the fireworks where she is ignored by all her friends.
Kakeru has many internal monologues with different instances of himself, showing his internal conflicts. He also sleeps outside in a tent as it gives him a sense of safety and belonging, which he usually lacks because of his family's constant moving.
Yanagi takes up running in frustration of her unrequited love
It's interesting to note that Sachi and Hiro, who do not dwell on their friend's potential reactions to their relationship have an unwavering devotion to each other (apart from Sachi's misleading which causes a small stir).
And it is true that this was a turning point in all their lives: Touko's life was disrupted by the visiting stranger, Sachi & Hiro are now in a loving relationship, Yanagi has left for her modelling career, Yuki has stopped running and has split off from the group. Kakeru also has travelled somewhere new, but experienced the same conflicted emotions that Touko did over the summer
If one is looking for a meaning in Glasslip, I would say it is to not worry about other people's perceptions of you, that growing up is a confusing, emotional time and that you find who your true friends are during testing times.
I recommend reading these reddit analyses of the final episodes: