Technical analysis is the practice of reading a chart of past prices and volume to form a view about future price behaviour. It assumes that everything known about an asset is already reflected in its price, that prices move in recognisable tendencies, and that crowd behaviour repeats often enough to be worth measuring.
In practice it is a toolkit, not a theory: trendlines, support and resistance, candlestick shapes, and indicators like rsi or macd. None of them predict. The honest framing is conditional probability: after a certain pattern, a certain outcome has happened somewhat more often than chance in the data you tested.
Its weaknesses are real. Patterns are identified after the fact, most studies of them fail to survive out-of-sample testing, and any edge that becomes widely known tends to shrink. Treat technical analysis as a way to structure decisions and define risk, not as a forecasting machine.
Related: price-action, chart-clutter, overfitting, indicator-lag, random-walk-hypothesis