A value stock is one where its current price is perceived to be different than its true market value and therefore is likely to return to what the true market value is and this can be either under priced or over priced.
If under priced the value would be gained in buying the stock with the intention of exiting when it returns to the true market value and if over priced the value would be gained in shorting the stock with the intention of exiting when it returns to the true market value.
Paul