Media buying refers to buying a particular space in a particular media. For instance, on radio and TV, media buy can refer to time you buy to advertise your company. In case of magazine, newspaper or websites media buying can be the space that you buy. For instance if you buy the front page of the newspaper to give your advertisement that is the media buy. If you choose the space for banner ads on a website that that is the media buy as well. so it is important to choose the right space or time for your ad as that would help you to capture a better market.
In terms of radio and TV ads, the prime time is considered to be the best as that is when the advertisers are able to reach out to more people. However, they also need to choose the ad space on the basis of the target consumer. A website meant for children should buy the banner ads in a children website as that would help it to better quality traffic. The children would be diverted to the advertised website. The ad in this case should be easily visible to the target client so that they may click on it and are diverted to your site. Thus media buying is very important for the advertisers.