It's used to indicate the absence of something, similar to how "nothing" is used in English.
Here are a few examples:
* "Onnume illa" (Tamil) - "There is nothing" (English)
* "Naan onnume panna matten" (Tamil) - "I did nothing" (English)
* "Unnaku onnume theriyaatha" (Tamil) - "You know nothing" (English)
Let me know if you'd like more examples or have any other Tamil words you'd like translated!