Some technical indicators (e.g. Williams %R) indicate whether the market is overbought or oversold.
Every time a stock or commodity is bought, it is also sold. And vice versa. So how can anything ever be over-bought or over-sold?
But I'm sure I'm missing something. What is it?
You're thinking of this as a normal purchase, but that's not really how equity markets operate.
First, just because there are shares of stock purchased, it doesn't mean that there was real investor buyer and seller demand for that instrument (at that point in time). Markets have dedicated middlemen called Market Makers, who are responsible to make sure that there is always someone to buy or sell; this ensures that all instruments have sufficient liquidity. Market Makers may decide to lower their bid on a stock based on a high number of sellers, or raise their ask for a high number of buyers.
During an investor rush to buy or sell an instrument (perhaps in response to a news release), it's possible for Market Makers to accumulate a large number of shares, without end-investors being involved on both sides of the transaction. This is one example of how instruments can be over-bought or over-sold.
Since Williams %R creates over-bought and over-sold signals based on historical averages of open / close prices, perhaps it's better to think of these terms as "over-valued" and "under-valued". Of course, there could be good reason for instruments to open or close outside their expected ranges, so Williams %R is just a tool to give you clues... not a real evaluation of the instrument's true value.