This is a very good question. The Mahabharata, when mentioned in passing, is often described as one of the two great Indian epic poems, the Indian answer to Homer's Iliad. A comparison with the Iliad is not a bad starting point. The Mahabharata, like the Iliad, is a long and complex work of epic poetry, the origins of which are shrouded in mystery. It probably took many centuries, perhaps over a thousand years, to reach the form in which it can be read today. Also like the Iliad, much of the Mahabharata is devoted to heroism in battle.
Perhaps the most striking difference between the two epics is their length. The Mahabharata consists of 200,000 lines of verse, interspersed with substantial passages of prose. This makes it about twenty times the length of the Iliad. It is also a vastly more varied text, including long passages of philosophy and theology and debates like Socratic dialogues in which characters discuss the correct course of conduct. Although the title Mahabharata refers to the Sanskrit text, there are also various other regional versions, including at least two from Indonesia and one in Tamil. The existence of these texts demonstrates the cultural and religious centrality of the Mahabharata, not only in India but in all the countries where India has been a major cultural influence.
The Mahabharata, therefore, might be described as the great foundational text of Sanskrit literature, Indian culture, Hindu religion, the longest epic poem in the world, and a compendium of Hindu philosophy and theology.