Buy things when they are cheap, not when they are expensive
The stock market moves up and down based on the performance of companies and the feelings of investors. Sometimes shares are priced too high or too low. Often the stock market moves as a whole, not based on the merits of individual companies. By paying attention, value investors can buy shares when they are temporarily cheap.
Another aspect of value investing is holding onto shares for the medium- or long-term. Assuming you have bought quality companies at a good price, holding the share over a period of years or even decades can provide you with dividends and capital gains. Trading less frequently also reduces your costs.
The most famous value investor is probably Warren Buffet, and he is also a great writer. His annual letters to shareholders are extremely readable, and there is a great collection of his essays available too.