A tarka is a (usually) very fast frying of spices/garlic/chillis and other ingredients in various combinations. The oil and fried spices are then added to the curry at the very end of cooking to preserve their freshness, and mixed through. This is also called tempering.
So a tarka daal, for example, is a lentil curry (the daal) which has had the tarka added at the end and mixed in for extra flavour.
YF