Goblin Guide- 1 mana for a 2/2 is already pretty good in an aggro deck, but throwing haste on top of it makes it incredible. Guide is almost always our preferred turn 1 play. He puts on a lot of pressure and gives us information as to what our opponent will be drawing into or what they have in their hand. It will sometimes give your opponents a land, but usually you have around a 60-66% chance of them revealing a non-land card. (that's an average of 20-24 lands)
Monastery Swiftspear- Like guide, but with prowess! Unfortunately she has one less power, but a few extra spells and she can get up to goyf sizes. Great early game, but you've gotta keep the tempo going with her on the field. Using your mana poorly can lead to her not dealing quite as much as she's capable of. Forces you to play a little more proactively, so keep that in mind. Use sorceries before combat and instants during combat.
Eidolon of the Great Revel- A controversial card in burn among people who are new to the format, let me explain this one to you. Yes, I hit myself with all my spells, and yes I'm okay with that. Modern is a format where the vast majority of spells are less than 4 CMC, so you end up with a bunch of decks that absolutely must answer an eidolon as fast as possible, otherwise they're dead. Burn decks aim to be the aggressor in most matchups, and so having one card that throws your opponent so far onto the back foot until they can kill it is really important for the deck. Burn aims to land threats that your opponent must answer in order to A) deal damage to them and B) to stall the development of their board state while using their mana to answer threats. Eidolon dishes out damage better than a lot of cards in the deck and as such is a real must answer card. He can also just simply shut down certain more niche decks, like storm or combo elves.
Grim Lavamancer- Another card that a lot of people don't like because he's too "slow". Sure, lavamancer would be a hell of a lot better if he had haste, but let me tell you how great it is to exile two card types from your graveyard and ping down a 3/4 goyf with this thing. It gets around the problem a lot of burn decks have with larger creatures in needing to 2 for 1 themselves to kill something big, like a goyf. It's also a great way to aim some board control at your opponents creatures while slinging your better spells at their face. When they have no creatures, he's just a way to gain card advantage by not needing to use cards in your hand to deal damage. Plus he can get in the damage without attacking. Super versatile in the deck, he's a must include. Definetely a better card slightly later in the game where you can activate him while playing your tempo.