To my understanding, EULA stands for End-User Licence Agreement, and in minecraft terms is a legal contract between Microsoft / Mojang and the user that purchases their minecraft account. It is an agreement that the user agrees to abide by, and if broken Mojang have the right to suspend your minecraft account, and if necessary involve legal authorities. The term is often referred to on our forums in regards to Cubecraft's lack of advantageous donator perks, as the EULA states that minecraft servers are not legally allowed to restrict or distribute in-game items or content to paying donators (i.e. kits, items, games, potion effects, experience, points) and is why Cubecraft is not legally allowed to provide donators with in-game benefits. This however only applies to items that provide a direct advantage, and therefore vanity items (such as lobby armour, gadgets and /fly) are allowed.
If you would like more information regarding Mojang's EULA, you can read about it here;
https://account.mojang.com/documents/minecraft_eula