Is it necessary to take Adwords certification for ads on Youtube?

No, to apply for monetization (and have ads attached to videos), creators must have tallied 4,000 hours of overall watch time on their channel within the past 12 months and have at least 1,000 subscribers.

And What is Adwords certification ?

I think the people who see most value in it is clients. If a client is working with a 'Certified Google Adwords Professional' this can give them a bit of confidence in who they're putting their trust in. However, most clients don't even know it exists, so not seeing it does not mean a client will say to you "I'm not working with you unless you're certified!". Further to this, I'm sure very few clients are checking you actually have taken the exam as it's not well-known enough to most of them - this does de-value it a bit in my opinion.

Having the qualification does not mean:

You will be successful at Adwords
You can make any Adwords account profitable
You can make money as a freelance Adwords manager
You can get a job at a PPC agency
You're an expert at Adwords
You've made it
You'll get asked to speak at PPC/SEM conferences
You should boast about it to your colleagues

Not having the qualification means:

You're as good at Adwords as your experience dictates
You can probably get the same Adwords jobs as you could before you took the exam.
You can probably get the same freelance Adwords clients as you could before you took the exam.
You're at just the same level of expertise as you were before you took the exam.
You may learn something from studying for the exam - depending on you existing knowledge.
Your LinkedIn page will contain one less certification
You don't need to prove anything to anyone!

It's also different things to different people. If you're an experienced Adwords manager, do you need to prove it with a certificate? (obviously I did - I'll print it out and put it on the fridge :) If you are at that level, you shouldn't have to study for an exam like that, so you haven't really learned anything.

But, if you're at exec level or a marketing intern at a PPC agency, studying for certification is a great way to prove to yourself and your employer that you understand the Adwords platform. You will learn things, you should be better at your job by the end of it - so why not? It's not, however, a substitute for experience getting your hands dirty in real-live Adwords accounts.