It is a contraction of “I would”. That being said, the meaning is *not* clear without context. It could be a conditional (I would…, if….), the future of the past (probably not in this particular sentence), or perhaps a synonym for “used to” (past habitual action that is no longer true in the present). The last option strikes me as particularly likely, but we’d need to see the sentences that came before it.

would. What would you do? I'd .....

In addition although a native speaker would never contract "I could" to "I'd" you may hear "I c'd" in casual spoken English.

The answers you've gotten are all good (Phil's in particular), and Frazier is right -- we don't contract "I could." If you see "I'd," it will always mean either "I would" or "I had."

I don't even think we contract "I could." That's not a thing. That said, I understand your confusion, especially without context. This sentence means that this person is able to imagine how that person used to be in whatever context they were presented. That explanation might not be helpful though.