I believe in weak stock market efficiency. I believe that the market does a good job of factoring in news and conditions but that the "wisdom of crowds" is far from perfect. There are plenty of valuing weaknesses that can lead to inefficient pricing and opportunities for gain. The simplest of those are spotted and then adopted by enough money that they become efficient and don't allow significant gains.
And a big problem for investors is that while I think there are plenty of inefficiencies to take advantage of finding them and investing successfully is quite hard. And so most that try do not succeed (do not get a return that justifies their time and risk - overall trying to take advantage of inefficiencies is likely to be more risky). Some Inefficiencies however seem to persist and allow low risk gains - such as investing in boring undervalued stocks. Read Ben Graham's books for great investing ideas.
Related: Market Inefficiencies and Efficient Market Theory - Lazy Portfolios Seven-year Winning Streak - investing in stocks