Media planning is the process of planning a for a media buy.
There are three primary considerations when it comes to media planning:
A media plan answers the question of who the target customer is. Additionally, it addresses how customers differ by advertising medium. These differences may range from geography and demographics, to interests and behaviors.
You need to know which type of ad medium to buy, and across what channels and/or platforms. You also need to determine how much of your budget will be allocated to each placement. Additionally, consider when your target customer may be actively engaged and paying attention to your media on each individual placement.
Last, how much of your budget will be spent on guaranteed placement versus RTB (real-time bidding)? This is along the lines of deciding between CPM and CPC bidding. If you decide to go the guaranteed route, define how much visibility you want at what price.
Once you have your media plan, you can pass it off to your media buyer. They will then usually manage the media buying process, from start to finish.