hops?

In the end, President Obama embraced the report and has begun to implement its policy recommendations selectively. In January 2014, he required NSA analysts to appeal to the Foreign Intelligence Surveillance Court before pulling records from the phone records database and narrowing the number of associated phone numbers, or hops, they can then look at from three to two.

What does the last part "narrowing the number of associated phone numbers, or hops, they can then look at from three to two" mean here?