What Is Mean By Schedule Lines

Schedule lines are created against schedule agreements. Say if you have a material which is procured from subcontract. If you want the delivery in particular days with schedule you can maintain schedule agreeement. In source list you will maintain vendor, schedule agreeement with validity dates and which agreement is relevant for mrp.
If you maintain 2 against the sch agreemetn mrp will generate schedule line which you can see in md04. Also this delivery schedules are updated in scheduling agreement (t cod ME33). When running mrp in initial screen delivery schedules--you have to maintain 3 create schedule line
Schedule line in opening period - Opening period is maintained in Configuration of "Schedule Margin Key" which is getting assigned to Material master in MRP2 view.
Your understanding of the result after MRP is needed.
Define Floats (Scheduling Margin Key)
In this step, you specify the floats for determining the basic dates of the planned orders. The floats are allocated to the material via the release period key in the material master record.
Opening period
The opening period represents the number of workdays that are subtracted from the order start date in order to determine the order creation date. This time is used by the MRP controller as a float for converting planned orders into purchase requisitions or into production orders.
Float before production
The float before production represents the number of workdays that are planned as a float between the order start date (planned start date) and the production start date (target start date). On the one hand, this float is intended to guarantee that delays in staging a material do not delay the production start. On the other hand, the production dates can be brought forward by means of the float to cope with capacity bottlenecks.
Float after production
The float after production should provide a float for the production process to cope with any disruptions so that there is no danger that the planned finish date will be exceeded. You plan the float after production between order finish date (planned finish date) and scheduled end (target finish date).
Release period
The release period represents the number of workdays that are subtracted from the order start date in order to determine the production order release. The release period is only relevant for production order management. Recommendation
The opening period should reflect the processing time the MRP controller needs to convert planned orders into purchase requisitions or production orders. The opening period should be at least as long as the interval between two MRP intervals,