The transaction efficiency shows how much was transferred in each tranaction on average with a fee of 1 USD. To remove the large variations from day to day, this chart shows the 2-week moving average.
To find the transaction efficiency, the adjusted total daily amount of Bitcoins (converted in USD with the daily price) is divided by the total amount of fees (converted in USD). We use the adjusted total amount transferred to get a meaningful estimate of the transaction volume with the noise removed. To learn more, see the dedicated article on Coinmetrics.
It is important to note that Bitcoin fees depend on the size of the transaction in bytes. The amount of Bitcoins transferred does not matter in the fee calculation. It is also possible to set the fee to any arbitrary value. Miners with confirm the transactions with the largest fees in priority. If the fee is too low, the transaction will not be confirmed in a reasonable amount of time. If it is too high, it is wasted.