Hendiadys is when two words that are supposed to be separate are squished together to make one word that means the same thing.
For example, if someone says "please give me a hand and help me lift this box," that's hendiadys because "give me a hand" and "help me lift this box" mean the same thing. Instead of using two separate ideas, they used one word to express both of them.
It's like if you wanted to order pizza and soda, but instead of saying "I want pizza and soda," you said "I want pizzoda." It's not a real word, but it means the same thing as pizza and soda together.
People use hendiadys a lot in writing to make their sentences more interesting or to convey their point more clearly. But it's important to make sure that the new word still makes sense and that readers will understand what you're trying to say.