Why is this advert appearing on webpages?

[Martin]

For the past week or two I've noticed that a certain advert keeps appearing when I go onto certain websites such as Youtube, Codecademy etc; the advert is of a 1 minute long Youtube video that I have to watch in order to carry on with what I was doing (I have attached a snip of the video in this question.) I don't watch the whole video as it's just an advert- it doesn't appear a huge amount however it's frustrating and I was wondering if anybody else is encountering this and is there anything that I can do to prevent this.
Any advice will be appreciated.