When we chant harinaam along with chintan (contemplating the pastimes of Krishna and Radharani) it is called Shuddha naam. We have to think only of Radha Krishna and no material subject. Such kind of chanting provides the fruits of chanting harinaam very quickly. "Anya chinta rahit". That is, without any other thought. If we give our full concentration towards harinaam, we will get the fruits of chanting harinaam very quickly.
Now what should we contemplate while chanting? There are many kinds of Chintan for different people of different capacities. For example, those who have come to Braja Dham to do bhajan, they may practice contemplating the Ashtayam lilas of Radha Krishna (ashtayam = eight three-hour intervals or the eight kinds of pastimes throughout a day) along with harinaam. Those who are incapable of doing so, may be due to a lot of work they are incapable of sitting at one place and do Ashtayam lila chintan, they may practice Naimittik lila chintan. It means contemplating the pastimes occuring right at that moment, as long as they chant sitting at one place. Suppose you are not able to contemplate the pastimes, or don't know much about the pastimes, then you can just contemplate Radha Krishna seated on a bejewelled gold bench in the Nikunja bhavan, in the deep forests of Vrindavan. They are laughing and joking with each other. Contemplate this scene. Otherwise, think you are serving Radharani. Suppose it is summer, you want to offer Radharani something that will soothen the summer heat. In a bejewelled plate take the dish you like most, or the dish you want to offer to the Yugal kishore and contemplate that now you are feeding Krishna with your own hands, then offer the adharamrita of Krishna to Radharani. Then bring some cool water flavored with camphor or kesar and offer to Radha Krishna. Then clean their mouths and hands with water and dry them. Chant along with this contemplation. If you are incapable of doing such chintan, just contemplate that Yugal kishore is seated in front of you and you are taking Darshan of them and they are looking at you with merciful eyes. If you are unable to do this, then just concentrate on harinaam you are chanting. Give your full attention to it and don't let anything else enter your mind. This is Shuddha naam. And if your mind us wandering to various thoughts, to Delhi, Mumbai, Kolkata, etc. and also chanting to complete the number of rounds you are supposed to finish then it is called Namabhas. It will not go futile but it will take a long time to achieve the fruits of chanting harinaam.
(Someone asks whether it is possible to achieve Radha Krishna's Prem practising only Namabhas?)
Practising Namabhas it slowly purifies one's heart and slowly the sadhak is able to beget the full mercy of harinaam. Then slowly he starts experiencing the pastimes of Radha Krishna as if he is present there. Slowly he is able to comprehend his actual form, that is the Manjari Swarupa. And serves Radha Krishna in that swarupa. So it is not that practising Namabhas results in Prem. But practising Namabhas slowly purifies one's heart so that the sadhak is able to chant Shuddha naam and then Shuddha naam gives Prem. First the sadhak enters and ascends through the stages of satsang, bhajan kriya, anartha nivritti, nishtha, ruchi, aasakti, bhav and lastly, Prem. (Please refer Stages of Sadhak). It is not possible by the hardwork of sadhak to ascend through these stages, but it is harinaam that takes you through. You cannot deliberate yourself from the stage of ruchi to the stage of aasakti. It is possible only by the mercy of harinaam. Naam prabhu is not just a word, it is the same as naami, Krishna and holds all the powers of Krishna. That is why even Namabhas purifies one's heart and gives him the ability to chant Shuddha naam and gradually gives Prem. But yes, you have to beware of Naam aparadhas.