The vaghar is the penultimate step in making dal. It's a spicy, smoky oil that you dump onto the top of the dal to round out the flavor. Here's the recipe I know:

1/2 tsp. black mustard seeds
1 tbsp. olive oil (or ghee, or whatever oil you want)
1 chile (jalapeño, habañero, serrano, adjust for your own taste in spiciness)

Put it all in a small pan and cover. Heat until the mustard seeds pop. Dump onto the top of your dal and cover. This will allow the smoke to settle into the food, so you'll get a nice smoky taste along with the spiciness.