Is it good to use Arabic when explaining English lessons?

Yes, but you must keep it to the minimum in order to make the mood of the class just English so students will concentrate on English more. But if you failed in making the students get the meaning or the point in English in any way then you can use the Arabic.

It is not good; actually is is a bad practice conducted by some teachers.

If you use Arabic in teaching English or any other foreign language, then do only expect bad performance and results! To get good results you need to teach a language using the same language as an instruction medium.