what does bin _time span does here?

Hi, I am having a bit of difficulty understanding what does **bin _time span** does here. Below is query shared in splunk community to find request per min by OrgName per day index=data earliest = -1d| bin _time span=1d | streamstats count as Req by OrgName, _time | eval requestsPerMin=Req/24/60 | eval requestsPerSec=requestsPerMin/60 | stats avg(requestsPerMin) as avgRequestPerSec, max(requestsPerMin) as peakRequestPerMin by OrgName, _time My confusion here is what bin _time span does. I want to find requestPerMin for Last week and month wise too. Could you someone explain how it can be done ?