A Pledge on Vocal means recurring encouragement for your favorite creators, and more stable, reliable support from your fans. Vocal+ members have the ability to collect $2.99/month Pledges from audience members who want to support their stories. As a Vocal+ member, you'll have the option to enable Pledges from your Settings page in the Creator Tools section. Once enabled, your account will be able to accept Pledges from your fans.
You can learn more about Pledges here.