📖What are Bollinger Bands? (from Oleg Polunin's website)

Bollinger Bands (BB) is a volatility indicator for identifying extremes on the price charts of various assets. They can be used to analyze trading volumes, market momentum, and statistical data. The proximity of the price to the upper line indicates overbought, to the lower line - to oversold. BBs do not give explicit buy or sell signals, but help to determine the direction and possible momentum of future movement.
Bollinger Bands allow you to determine the average volatility for a particular asset and identify potential price highs or lows based on standard data for the previous period. BBs are also suitable for detecting price patterns such as "double tops" or "double bottoms".

On the chart, "BB" is represented as a "channel" that follows the price curve of an asset and consists of three lines: the average, the upper and the lower. The basis for plotting the BB is the middle band, the Simple Moving Average (SMA). The other two lines indicate positive and negative deviations from the SMA.
The standard settings for the BB indicator are the 20-SMA period and the deviation of 2. For the daily chart of ETH or BTC, the settings would be 20-SMA with a deviation of 2.
An increase in the period may lead to a decrease in the sensitivity of the indicator. When fine-tuned, the asset's price curve should be between the lower and upper lines 90% of the time.

Cons of BB
1. Low volatility can make them worthless, enough activity is required to form a channel.
2. Decisions based on BB alone can be risky as the indicator does not give buy or sell signals.
3. A breakout of the BB borders indicates the continuation of the trend, but does not always give trading signals.
4. It is not recommended to combine the BB with other indicators based on moving averages as recommended.
5. Using Too Long Periods Makes the Indicator Less Sensitive and Creates "Stability"