If you had never seen Hawaii's state flag before, you were probably a bit confused upon first glance. Many people end up asking the very valid question, "Why does Hawaii's flag have the British Union Jack?"
Hawaii's flag is actually the only state flag to have symbols of other nations on it.
First, the eight alternating white, red, and blue stripes pay homage to the eight major Hawaiian islands (their colors have no meaning). But you're probably here to learn about the other part.
In the canton of the flag, you find what we vexillologists call a "Union Jack". It is the national flag of the United Kingdom of of Great Britain and Northern Ireland to be exact, and even though the US threw off the chains of British rule in 1776, it's flag once again flew over US soil when Hawaii became a state in 1959.
You see, in 1793, Captain George Vancouver visited Hawaii for the second time and presented Kamehameha I with a Red Ensign. This red flag with a Union Jack in the canton was used unofficially as the flag of Hawaii until 1816.
Then Kamehameha purchased a ship from the British East India Company and named it Kaahumanu. As part of this purchase, the British East India Company's Ensign (which looks very similar to today's Hawaiian flag) was flown from it.
Hawaii was a British protectorate for a long while, and the flag the Hawaiian Kingdom has always had Union Jack on it since 1793. From 1816 on, versions of the flag with different orders of the stripes was flown but it hasn't changed much since.
Then came the USA.
How Hawaii became a US state (clears throat)...
sigh...here, this TED-Ed video says it all:
Basically, a bunch of white christian missionaries got the USA to rise up a coup and overthrow the sovereign Queen. In 1993, the USA formally apologized, saying:
...the overthrow of the Kingdom of Hawaii occurred with the active participation of agents and citizens of the United States and [...] the Native Hawaiian people never directly relinquished to the United States their claims to their inherent sovereignty as a people over their national lands, either through the Kingdom of Hawaii or through a plebiscite or referendum.
Soooooo, yeah...not great.
In fact, the Hawaiian Sovereignty Movement now flies the Hawaiian flag upside down showing that they are still a sovereign nation, and they are under distress.
In 1990, the governor declared July 31st to be Hawaiian flag day, or Lā Hae Hawaiʻi. This is the same day as Lā Hoʻihoʻi Ea, or Soverignty Restoration Day celebrated by the Hawaiian Sovereignty Movement.
So even though the flag of Hawaii looks similar to other places that the British colonized, it actually is a proud symbol of Hawaiian sovereignty. There are other flags that celebrate Hawaiian culture like the Kanaka Maoli flag, but this one (flown upside down) is the main symbol of the Kingdom of Hawaii.