AdMob is an ad network. You as a publisher can earn money by displaying ads delivered from the network. You’ll be paid from AdMob without any direct contact to the advertising companies.
DfP is an ad server offered as a cloud solution. You can set up an instance yourself but it’s empty. It contains no ads. You’ll need to go out, acquire ad campaigns from advertisers, book the campaign into the ad server and display the ads in your app. You’ll be in direct contact with the advertising companies and get the money directly from them.
For simple monetization, go with AdMob. If you have a very popular app and depending on the market you’re in, you might want to set up a sales team that acquires premium ad campaigns. Then you can use DfP for managing your premium campaigns and even configure it to use AdMob for ad impressions not filled by your premium campaigns.