What is the RSI and how do you use it properly?

RSI above 70 = overbought, below 30 = oversold. But in a strong trend, RSI can stay above 70 for weeks. The real signal is RSI divergence — price makes new high but RSI makes lower high.

I use RSI divergence more than overbought/oversold levels. Bearish divergence at resistance = high-probability short. Bullish divergence at support = high-probability long.