The Frequency Cap will limit the number of times your ad is shown to the same person each day (in a 24 hour period). Increasing the frequency cap gives you access to more impressions, but some of them may be from the same user. Keep in mind that it can sometimes take multiple impressions before a user decides to click, so try opening up frequency cap if you want more volume.
We recommend launching a campaign with a frequency cap of 1. This allows your budget to be spent on unique users, as opposed to spending your entire budget showing a subset of users the same ad over and over again. This wastes your budget and hurts your campaign performance. When scaling, we recommend going no higher than a frequency cap of 3.