From Seo Wiki - Search Engine Optimization and Programming Languages
Frequency capping is a term in advertising that means restricting (capping) the amount of times (frequency) a specific visitor to a website is shown a particular advertisement. This restriction is applied to all websites that serve ads from the same advertising network.
Frequency capping is often cited as a way to avoid banner burnout, the point where visitors are being overexposed and response drops. This may be true for campaigns of a direct-response nature measured by click-throughs, but it might run counter to campaigns of a brand-building nature measured by non-click activity.