Explain forward filling and backward filling (data filling)
Yes you are correct. Forward filling and backward filling are two approaches to fill missing values.
Forward filling means fill missing values with previous data.
Backward filling means fill missing values with next data point.

You can refer the below url for more on missing values:

https://www.kaggle.com/juejuewang/handle-missing-values-in-time-series-for-beginners

https://www.kaggle.com/dansbecker/handling-missing-values

https://www.kaggle.com/meikegw/filling-up-missing-values