Adventures introduces Reserve cards. They go on a Tavern mat. They hang out there, turn after turn, waiting until you are ready for them. "Calling" one means moving it from your mat to your in-play area; then it gets discarded normally that turn, to eventually be drawn again and go back on the mat. Wait let's go over this again. You play it, just doing the part above the line, putting it on your mat. Later, you move it off the mat into play, just doing the part it says there, whatever it says for whenever it says. Okay.
Guide can give you a new hand at the start of your turn. You just skip right past a bad hand, or a Militia'd hand, or a hand that doesn't have that Hero you need to upgrade. With multiple Guides on your mat, you can keep going until you run out or find that good hand.
Duplicate duplicates something. That name just tells the whole story. You buy a Lab; you Duplicate it, two Labs. You gain a Gold from Hero; you Duplicate it, two Golds. Whatever you are getting, you probably want more of them. Late in the game, Duplicate Duchies.
Coin of the Realm is a weird one. When you play it it makes $1 and goes on your mat. On some future turn, you call it to get to play more Actions. So it's a village that you can't (normally) use the turn you play it. And it's a village that just waits until you need it. You draw your two Heroes together, play one, call the Coin, and play the other one. You can also use Hero to gain a Coin. Will the other Reserve cards all be combos with Hero? *checks...* No. Not all of them.