A niche basically means targeting a specific market instead of trying to sell to everyone. Like instead of making content for "everyone interested in cooking," you target "bachelors who want 5-minute meals in Mumbai." It's important because competition is less, you can charge better prices, and you build loyal audience.
In India, especially for YouTube and Instagram creators, niche is everything. People trust specialists more than generalists. If you're one of 1000 cooking channels, you'll get lost. But if you're THE bachelor's cooking channel, you own that space. This is especially useful for Indian creators since the market is huge but also very saturated.