Koinobori meaning "carp streamer" in Japanese.
Celebrated in honor of children for a good future and in the hope that they will grow up healthy and strong.
According to Japanese American National Museum, the carp was chosen as a symbol for Boys' Day because "the Japanese consider it the most spirited fish -- so full of energy and power that it can fight its way up swift-running streams and cascades. Because of its strength and determination to overcome all obstacles, it stands for courage and the ability to attain high goals.
(by wikipedia)
https://en.m.wikipedia.org/wiki/Koinobori