Can I understand in this way? Let me know if any statement is wrong or not accurate.
Reason of data filling:
Assume I have a consecutive data (e.g., daily log data), and partial data are missing. In order to make some calculation (e.g., mean value), we first need to assign values to the missing parts (e.g., equal to existing data)
Forward filling and backward filling are two data filling methods.
The difference is the filling direction? E.g., Tuesday data (missing) equals to Monday data (existing) is forward filling. The opposite is backward filling.