Reach and Frequency is more akin to traditional advertising than any other Facebook Advertising option. They enable you to book a specific audience you want your ad to be shown to, lets you put a cap on the frequency at which they will see you ad, and insures that those people will see it. 

It enables you to minimize the possibilities of under-pacing and under-delivery and lock in your cost per impression (CPM).
It's designed for wider audiences (at least 200,000 people) and companies that want to know their costs and delivery expectations upfront.
It's available for Traffic, Conversion, App Installs, Reach, Brand Awareness, Engagement and Video Views objectives. 

For reach and frequency campaigns, the cost is always CPM, meaning you'll pay per thousand impressions.

Warning: You can only target one country per campaign so if you want to advertise to multiple countries, you’ll have to create multiple campaigns.

Buying your campaign using Reach and Frequency is an alternative to bidding for ad space through the auction. It gives you more control over the number of people your campaign will reach and how frequently a person will receive your ad during your campaign.

You can create campaigns that range from 1-90 days and can schedule them up to six months in advance. Major holidays and sporting events can make reaching your audience more expensive, so you can plan ahead or avoid them for a more even pricing.

In a nutshell, it's a predictable and safer option to book an audience and get impressions on your ads.

Next let's see how you can create one!