Right effort (sammà vàyàma) is the sixth step on the Buddha's Noble Eightfold Path. Effort is the catalyst that brings the other qualities on the path into being and then animates them. Thus, for example, although we all have the potential to be mindful we may never have been until we heard about it and appreciated its value and utility. We might then make an effort to be mindful and become so. Having become mindful, at least for a while, we might then try to strengthen this ability and the depth of our mindfulness. Effort, determination and vigour are essential in every step of this process. The Buddha defined Right Effort like this: `And what is Right Effort? One generates the desire to prevent the arising of unskilful states not yet arisen, the desire to give up unskilful states already arisen, the desire to develop skilful states not yet arisen and the desire to nurture and further develop skilful states already arisen. One makes an effort, exerts energy, focuses and directs the mind to these ends'(D.II,312). 

However, the Buddha always pointed out that simply gritting one's teeth and straining can sometimes be counter-productive. Effort is to exert energy but Right Effort is more subtle and nuanced than that.  It is a little like rowing a boat. To get the boat where one wants to go is not simply a matter of pulling at the oars. Firstly the oars have to be pulled in unison. When we are going against the current we may really have to pull hard, when going with the current we may not have to pull at all. To keep going in the right direction we might have to pull  only on the left oar and at other times only on the right one. Thus to be `right', effort has to be balanced against other factors, its intensity must be adjusted according to circumstances and we have to know when to relax and let things unfold naturally. As the Buddha said, trying too hard results in agitation while not trying hard enough leads to stagnation (S.I,1). See Zeal.