What is a breakthrough channel on YouTube?
Breakthrough or Outlier channels are smaller or newer channels pulling far more views than their size would suggest. They typically have relatively low subscriber counts compared to their views, and often use formats or angles that feel less saturated than those of the authority channels.
However, the formats and angles can also be torched to death across many niches and it might be wise to consider avoiding them if you're entering the space too late. Something like "Why It Sucks to be" is the shape of one that has been.
What they tell you
Breakthrough channels tell you where the niche is expanding right now, what new formats the algorithm (read as: the audience) is currently rewarding, and whether there is still room for a new channel to enter. When a small channel gets huge views, that is almost always the algorithm signaling that demand exists, but supply is weak. That gap is exactly what you are looking for.
Watch how many of them are rising
However, you have to keep tabs on how many Breakthrough Channels are rising. Use your Discovery tool to track these, but if there are a lot popping up: How are the views looking? Rising, stable, or falling? The maturation of the market and the niche is important to assess as well. Entering too late in formats that have been heavily used is often a negative action for your success.
Ultimately, Authority channels show you what is proven. Breakthrough channels show you what is emerging. Both matter for your niche research but outlier channels are where the clearest signals of opportunity come from. Just try to be earlier rather than later.