What does it mean?
The personification of evil and temptation.
-from Access to Insight
Why does it matter?
Māra is, in a word, the devil. What’s unclear (at least to me) is whether Māra is a literal devil, or, as the definition above hints, more of a personification. It seems to me that many (most?) Indian gods and other spiritual beings have a literal side and a more metaphorical aspect, and one is more or less free to “take” them as literal as you like.
Māra comes up all the time in the Buddhist scriptures. Sometimes there appears to be many Māras, sometimes one. He is often shown in verbal combat with the Buddha, most famously on the evening before his enlightenment. Often all it takes to defeat Māra is recognizing him. Māra can personify death, fear, distraction, or almost any defilement.
Māra is sometimes hilariously human. Here he is feeling sorry for himself:
Māra muttered to himself, “For seven years I’ve chased the Buddha, trying to find an opening. Like a crow mistaking a rock for a juicy piece of fat, I’m constantly disappointed.” Overwhelmed with sadness, hit guitar fell from his arm. Then that disappointed little devil disappeared right there.
For me, I think Māra is a useful literary device, but of sometime he can feel so literal when you’re sitting on the cushion!