What is the importance of Bitcoin dominance?

I often hear people talk about the Bitcoin dominance and for those who watch Benjamin Cowen's channel, you'll know how much importance he attaches to it.

Pretend for a minute I'm a moron when it comes to Bitcoin: Bitcoin dominance means it's marketcap share compared to altcoins, am I right? The higher the dominance, the higher it's share. However I'm not sure why that's so important and what effect it has on the price of BTC or what it means for the bull (or bear) market. Can anyone ELI5 this for me?