mathmatics seva...

To write a good description, you first have to understand the purpose of your channel, so you can accurately describe it to others. Determine the goal of your channel. Do you want to just entertain people or do you want to inform them?

If you are in the health niche, you would definitely be making more informative videos. If you are a travel vlogger you would spend more time creating entertainment-style videos. And there are niches like catering and food that inform and educate, but usually the creator would lean more towards one goal. 

Understand your channel's purpose!