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.