In this beginner's guide, we try to stay away from spoilers as much as we can, but one of the most important plot points of Attack on Titan is actually the major spoilers, and to get rid of them would feel dishonest.
Therefore, if you want to enter this anime with only the essentials and be surprised when you leave, feel free to skip to the next section. For the rest of us we will spoil spoilers so our spoilers will feel really spoiled.
…Well, that word sounds weird now. I think we can continue
When all seems lost, Eren discovers a latent power lurking deep within him that could finally turn the tide of battle, reclaiming hope for humanity after a generation of tribulations—the unprecedented ability to transform into a Titan.
How and why he was able to perform this incredible feat was unknown even to him, and no one could be sure how safe or stable it would be. This of course puts him on a more even footing with his foes, and watching him squander swarms of Titan drones is quite a sight to behold.