you can think of Bitcoin as the first coin out there. It was basically the first coin that started using the blockchain, which is the technology that all coins on the crypto market are based on. Due to that, the fact that BTC has been the first, it has seen the biggest adoption from users and that's the reason why it has the highest dominance in the market. BTC is valued at about 120 billion at the moment and has about half the market share of all crypto's! That's quite something! Altcoins is anything else. Whenever you hear altcoin it's a coin that is not BTC essentially, since there can only be one Bitcoin and any other coin besides that is just an "alternative" or otherwise "Altcoin". Don't let that confuse you though, altcoins have just as much potential as BTC does and time will show that.