The KDJ is a variant of the stochastic oscillator, commonly used to analyze the strength and dynamics of a market in the short term. It consists of three curves, named K, D and J, each having a distinct role:

1) K: This is the "fast stochastic". It reacts more sensitively to price changes.

2) D: This is the "slow stochastic". It is smoothed to avoid too much volatility and give a more stable signal.

3) J: It is calculated from K and D (in general, J = 3*K - 2*D). It tends to rise higher and fall lower than the other two lines, which can highlight areas of excess (overbought or oversold).

Usual interpretation:

- When K and D evolve in the upper zone (often above 80), we speak of overbought.

- When K and D are in the lower zone (often below 20), we speak of overselling.

- The crossings between K and D can offer buy signals (when K goes above D) or sell signals (K goes below D).

- The J curve, by going well outside these zones, can highlight strong excesses, sometimes precursors of reversals.

As with most technical indicators, it is preferable not to rely solely on the KDJ. It works all the better when combined with other tools (RSI, MACD, volumes, etc.) and with a fundamental analysis adapted to the market context.