Yes it is inbreeding.
No, there's nothing inherently wrong with it.
With some recessive traits that people are unsure about, they breed back to the same line to "prove out" the trait as being genetic.
Technically "line breeding" is breeding to another animal in the same lineage(mom to son, dad to daughter, aunt to nephew, etc). If there is any unwanted genes, it can "double up" and be expressed or expressed more. If there are wanted genes, same principle.