Diegetic sounds is sounds that come with the natural part of the scene coming from sources within it. For example, if two characters are talking to each other, you would hear their voices from the conversation. Or if a character put down an object such as a hammer on a table, you would hear a bang noise. Or even birds singing in a forest. Each sound you would expect from looking at the environment or looking at what is happening in the scene.
Look at the image above. Think about what you may hear if you where actually in this. Water splashing on the rocks and into the water below, birds singing in the trees, maybe frogs croaking, leaves rustling in the wind. sounds that come natural and make you feel like you are actually there.