What significance does '86400' have in Splunk? For example, why is it used here, '| eval day=floor((now()-_time)/86400)'?
86400 is the number of seconds in 24 hours. In your example it is used to convert the number of seconds since _time into the number of days since _time.
86400 is the number of seconds in 24 hours. In your example it is used to convert the number of seconds since _time into the number of days since _time.