Explain Like I'm Five

It's important to know some not oft cited history on this topic. Key to understanding this conflict is some knowledge of the pre WWII history of the region.

First and foremost, understand that the word "Palestinian" is a very recent invention, and like almost all national identities, was created by a state. That state would be the UK when it governed what is now called Palestine, and preceding it, the Ottoman government when it ruled the region. Later, the national identity was crafted using propaganda by the entities that make up the Palestinian government. Prior to European colonialism in the region, most of the people throughout the Arab world regarded themselves as simply Arabs, and there were small ethic groups throughout such as Berbers in north Africa and several different ones throughout the Arab peninsula. The majority was Arab, a legacy of imperialism by caliphs in the region. These borders we see today, and the borders of the past, were/are amorphous, ever changing, and the national identities of all the peoples within these borders were crafted by their respective states to create easier to govern populations. Most Arabs identified with their family or clan, including what we call Palestinians today.

Now with that out of the way, a bit of history on the Israelis. "Zionism", which encompasses the concepts of Jews as a nation/race, subsequent nationalism and claim to Palestine is also relatively new, having emerged in the late 19th century, mostly in Europe. Most Jews are European, but there were significant populations in and around the Ottoman Empire including Palestine. I'm glossing over a lot, but the Zionist Congress began a propaganda push for Jews to move there from Europe which was successful, successfully lobbied the UK after the Palestine Mandate began into adopting the Balfour Declaration, subsequently there were conflicts predating Nazism between Jews and Arabs there, including what we today would classify as terrorist attacks by Zionist paramilitary groups.

It is important not to moralize this stuff too much. The Jews in the area were a marginalized group there during Ottoman rule, there were nationalist and socialist movements within Zionism, the British played a role, and virtually every national identity on the planet was created via armed struggle/terrorism, propaganda and/or state abuse of populations. That said, I think it is telling that the stuff I've discussed here is not common knowledge with regard to the history of the conflict we think of today.

If you want to dive down the rabbit hole that is pre-WWII history of this conflict that is not often talked about, I've found some links here that should get you started. There's a lot of important history that people should learn about.

https://en.m.wikipedia.org/wiki/World_Zionist_Congress

https://en.m.wikipedia.org/wiki/Balfour_Declaration

https://en.m.wikipedia.org/wiki/Bar-Giora_(organization)

https://en.m.wikipedia.org/wiki/Hashomer

https://en.m.wikipedia.org/wiki/Haganah

https://en.m.wikipedia.org/wiki/List_of_Irgun_attacks

https://en.m.wikipedia.org/wiki/Socialist_Zionism