This expression indicates that a person does almost nothing but V or that V takes place repeatedly. The use of ばかり, which is inserted between Vてand います, indicates the speaker's perception that the action/state is not desirable.
あの人はなまけてばかりいます。
That person always neglects his/her work.
マナさんはじゅぎょうのとき、まちがえてばかりいます。
Mana keeps making mistakes in class.