Proficiency is how good a learner is at the language they are learning.
Proficiency tests such as IELTS and Cambridge Main Suite examinations test how good a candidate's overall ability in English is.
In the classroom
The level of proficiency in English that learners want to reach varies considerably and depends on the reasons and motivations for study. Many learners say they want a very high level of proficiency but need to be guided by teachers to identify the level they really need, and also the areas they need to be proficient in.