The close date that you are asked to fill out is an expected close date, i.e. when you'd expect to win the opportunity. (Unless you close an opportunity as won or lost. Then it's the effective close date.)
Salesflare then calculates an expected revenue based on what's in your pipeline.
It sums opportunities with a close date in the selected time range, multiplied/weighted with the probability of each stage.
Don't want to enter close dates?
If you don't want to use this feature, go to "Settings" > "Configure pipelines". Click on the blue pencil icon to edit the stage in which you don't want to enter close dates anymore.
Input "0" under "Probability (%)" and save the changes by clicking on "Update".
You will not be asked for close dates anymore for opportunities you add to stages with 0% or 100% probability.
This also means expected revenues will no longer be calculated for opportunities in this stage.