Hello Ananya

Mock Tests are provided by National Testing Agency (NTA) for students to practice. Mock Tests are available in online mode. Tests can also be downloaded by the students and those who do not have Computer system are provided the facility to walk to the JEE Main 2019 Test Practice centres and practice the mock tests there. Test practice centre facility is provided by NTA for JEE Aspirants.

If your query is about Mock Seat Allotment which is conducted before the final seat allotment, firstly, JoSAA will allot the seats in two mock rounds. On the basis of mock rounds, candidates will be able to know about the seats they could be alloted on the basis of their filled choices.
Mock rounds help the candidates to know about the seat allotment process and also allow candidates to know and choose better preferences in the main rounds. Based on the mock seat allocation, candidates can change their choices of courses and institutes.
For example, if you think based on your score and mock seat allocation trends, you could get a better seat in a institute which you have not filled, you can fill that choice as well.

Mock Allotment Round 1 and Round 2 are to be held tentatively in the third week of June 2019.

Do let us know if you have any other query.
Good Luck!