The SLA log screen can be found in section Reports > Tickets > SLA log. This description of columns is also valid for the exported data from the SLA log.
Level - is a set of definitions - reply / resolve / pickup "maximum" time conditions, business hours and holiday exceptions, configured in Configuration > Automation > SLA > SLA levels
This SLA level is set by SLA rules when ticket is created, or later by some regular rule or manually by mass action.
According to definitions of SLA level some due time is set for the ticket - depending on the state of the ticket. If more than 1 condition of SLA applies on the ticket, the shortest one is displayed in the SLA log (all applicable "running" SLA are displayed in ticket sidebar SLA box).
SLA started (in export: date_created) - date and time when SLA was activated - usually time when ticket changed state.
Due date (date_due) - date and time when activated SLA condition must be fulfilled or ticket will overdue - according to its definition in set SLA level. In some cases, it can't be greater than SLA started, e.g. "first resolve" can be already overdue when ticket changes state to Open, because it is calculated from time when ticket was created through all opened states.
Closed (date_close) - real date and time of deactivating of "running" SLA - again usually time, when ticket changed state (was resolved, was replied, etc.)
SLA (sla) - is time remaining until due time, or time of overdue. In exported data negative value means time before due time, positive value means overdue.