Hey Hardik,

Mock Tests are the practice tests provided by conducting agency or other websites so that a candidate gets an idea of how the actual question paper is going to be. Usually, mock tests are available for Computer Based exams but some sites may even provide them for pen-paper based exams too. It has the same pattern as exam question paper just with fewer questions (generally) like what kind of questions are going to be in exam i.e. level of difficulty, negative marking (if any), different sections (such as physics, chemistry, and maths). 

For example, let’s say some entrance exam has 120 questions divided into 3 sections so the official website gave a mock test link where the test is of one hour has 30 questions (10 in each section). 

So, it is actually quite important to do some mock tests before giving an entrance examination. Especially, when you are giving engineering or medical exams because the level of difficulty can be determined. That is why exams such as JEE, BITSAT, VITEE have mock tests available on even official websites. 

Another important point to note about them is that THEY DO NOT CARRY ANY MARKS, they are just for practice.

If you are going to give JEE, then you can and you should give the mock tests. National Testing Agency (NTA) provides JEE mock tests which are just like the actual paper, it has 90 questions and is of 3-hour duration with negative marking. After completion of this test, you will get the result which tells greatly about where you stand, what are your strong and weak points and how much time you are taking in solving questions. Also, they provide the answer key of those questions. Downloading of the mock test questions can also be done by the candidates.

The link provided below is of the official NTA site where you can give the mock test for JEE-

http://data.nta.ac.in/Quiz

Apart from this, many other sites also provide mock exam for which you may have to register. You can also go for that.

I hope I was able to help you. If you have any other doubt or query, feel free to ask or comment.

All the best!