Is it necessary to include every task of the WBS in WBS dictionary? like if a sub task has no further explanation do i have to include it in the WBS dictionary?
Who is evaluating your WBS Dictionary? If your WBS Dictionary is a deliverable that requires "acceptance" from the buyer, then you need to meet the requirements of that buyer. If you are only concerned by the users of the WBS Dictionary and are confident you have no risk of some misunderstanding from a project team member if you exclude WBS elements from the dictionary, then you can answer your own question. If this is a homework assignment, then you need to read through your notes to see what your instructor told you and then follow that.
In practice, I include in the dictionary only down to the work package level, in which I have the loaded budget, resources, materials, dependencies, owner, etc., so that the work package owner knows what is expected. And the summary levels going up the WBS. I would alter that only if the buyer requires. Otherwise, no one else cares.