I saw this question in an application form, but couldn't understand what does it mean?
By when do you need a decision to join the next cohort?
In this context, a cohort is a group of people who join the course at the same time.
The website explains
Some individuals have visa requirements (which we can support with), or long notice periods. If this is the case please make a note here and we can expedite your application.
A decision, in this context is the decision that "Entrepreneur First" make about whether to accept or reject your application.
So it may be that you need to know the result of your application early, so you have time to apply for a visa, or tell your employer that you are quitting your job. If a visa takes 6 weeks to process (for example) you need to know the result of the application 6 weeks in advance, so you have time to get the visa. Here you can write the date by which you need to know the result of the application.