Originally used for commodities, the Stock-to-Flow model is also used on Bitcoin and describes the scarcity of a good. The stock represents in this case the total quantity of BTC tokens and the Flow represents the quantity of those tokens available on the market during the analyzed years. Therefore: Stock-to-Flow = Stock/Flow.
The colors used on the graph match the remaining days until the next halving.
A Stock-to-Flow which increases indicates a scarcity of the asset, which can cause an increase in the price due to the supply and demand law.
This indicator was very relevant in the past (a bit less nowadays), by matching the price of Bitcoin. It can therefore give indications about the future price of the famous cryptocurrency.