Theoretically, mainstream media is supposed to show us the most important news only, but generally this idea has been less trusted. Mainstream media is made by huge corporations that receive enormous amounts of money (sometimes from governments, sometimes from private businesses, but always from someone with interests) and often report news in a way that can be considered not completely honest or neutral.
Independent media is supposed to be a response to that. Many claim to be free from that problem because they do not receive that money. Some claim to be neutral, but many claim to be left or right leaning so people will know upfront that their news also carry their opinions with it.