The Iliad is an ancient Greek epic poem, typically attributed to Homer, that provides an account of a few weeks in the final year of the Trojan war, which was the ten-year siege of the the ancient city of Troy by an alliance of Greek city states.
The Iliad is among the oldest surviving masterpieces of Western literature (dated to around 800 BC), and is worth reading due to its enormous influence on Greek and Roman thought, and on subsequent societies influenced by the Classical world. The Iliad is also a key source of our historical understanding of the age of Homer (around 800 BC) and of the events described (around 1200 BC). In that sense, the Iliad is the prep work for everything that follows.
Reading the Iliad can be daunting due to its archaic setting and length. However, it is not as hard as it might seem! It should be remembered that these stories were meant for public consumption to begin with. There are literally hundreds of translations of the Iliad you could pick up today, but in the end, the best translation of Homer's epic poem is the one you actually read. If you're a first-time reader and are looking for a more approchable version of the Iliad, I highly recommend the 1 Robert Fagles translation. Of course you're going to miss a lot of depth and nuance on your first reading, but that's fine. For a deeper second read, you could try the slightly more challenging 2 Richmond Lattimore translation along with a companion book such as Willcock's 3 A Companion to the Iliad (which is based on Lattimore's translation). Finally, for some additional background on the Trojan war, Barry Strauss' 4 The Trojan War is a good read.