This is a way to answer a question or give an opinion, when you're not really sure:

I'd say she'll probably be back at about 2:00.

"I'd" is short for "I would". So you can also say "I would say ___":

I would say that you can probably find one for under ten thousand dollars.

You can introduce the rest of the sentence with "that":

I'd say that I'm two or three weeks away from finishing.

"I'd say" can also go at the end of a sentence:

It takes about an hour and a half, I'd say.