"Altcoin season" refers to a market phase when alternative cryptocurrencies (altcoins) outperform Bitcoin in terms of price gains. If you're saying #AltcoinSeasonComing , it suggests you're expecting a surge in altcoin prices soon.
Here are some signs that often precede an altcoin season:
Bitcoin dominance decreases (i.e., altcoins gain a larger market share)
Ethereum gains strength vs Bitcoin
Low-cap coins start showing explosive growth
Retail interest and social media buzz increase