What the chart shows
Each month starts with the close-to-close change: Δₜ = Closeₜ − Closeₜ₋₁, Gainₜ = max(Δₜ, 0), Lossₜ = max(−Δₜ, 0). The seed averages are the simple means of the first 14 gains and losses. Wilder smoothing then applies AvgGainₜ = (13 × AvgGainₜ₋₁ + Gainₜ) / 14 and AvgLossₜ = (13 × AvgLossₜ₋₁ + Lossₜ) / 14. Finally RS = AvgGain / AvgLoss and RSI = 100 − 100 / (1 + RS).
How to interpret it
Readings above 70 are traditionally treated as overbought and readings below 30 as oversold. On a monthly timeframe the signal moves slowly and becomes final only when the month closes. RSI measures momentum strength; it does not identify an exact reversal price or date.