“I guess it requires some assembly.”

You bought an office chair online. You thought that it would be sent to you completely built, but instead you were just sent a bunch of parts that you have to put together. You say to yourself:

I guess it requires some assembly.

I guess (something is true)

Use "I guess..." to express that you've just started to think something. For example, you can use it when:

• you read the instructions for something, and now you think you can explain how to do it
• a friend has been trying to convince you to go to a party, and you've finally been convinced to go
• you've been waiting for a long time for someone to call you, and now you've decided that they're not going to call.

Here are some examples:

Hmm. They're not here. I guess they got tired of waiting.

I guess I might as well stick around and have one more.

I guess so.

(something) requires assembly

To "assemble" something means to put it together from a bunch of different parts. "Assembly" is the noun form of "assemble".

So saying that something "requires assembly" means that you have to put it together yourself. Sometimes packages for products like furniture or children's toys will have a label that says: 

Some assembly required.