This is the bread-and-butter for quant funds. AQR has pioneered this space the most.
Multifactor models are models designed to extract cross-sectional risk premia in markets. Put simply you look at all the stocks in a market. Rank them based on some metric(s). Long the top decile. Short the bottom decile. If that market-neutral portfolio outperforms on a consistent basis then well done! You have found a risk premium.
Classical and well known risk premia include:
Momentum – stocks that moon continue to moon.
Size – small caps tend to outperform large caps.
Value – high-value stocks tend to outperform low-value stocks.