Frequency capping controls how often one person sees your ad. If someone keeps seeing the same phone ad 20 times a day, they'll get annoyed — and you'll waste money showing ads to someone who's clearly not interested. Frequency capping solves this by setting limits, like "show each person this ad maximum 2 times per day and 5 times per week."
This feature is available on virtually every major ad platform — Facebook, Google, YouTube. The benefits are twofold: customers don't get irritated by repetitive ads, and your budget goes further by reaching more unique people instead of hammering the same ones. It's especially important for awareness campaigns where broad reach matters more than repeated exposure.