BGP communities are attribute tags that can be applied to received or advertised prefixes. They are defined by three main RFCs:
Communities can be used to tell other autonomous systems what to do with these prefixes. For instance they can be used tell an AS to restrict advertisement of a prefix to only Europe peering customers. Most good network service providers publish a list of communities they will honor. Note that some communities are reserved. Blackholing IPs, for example, can be achieved with RFC7999 community.
In Peering Manager
Inside Peering Manager, you create communities to apply them on autonomous systems, BGP groups, Internet Exchanges or BGP sessions. For each community that you create, the following properties can be configured (n.b. some are optional):
Name: human-readable name attached to a community.
Slug: unique configuration and URL friendly name; usually it is automatically generated from the community's name.
Value: actual value of the community.
Type: when to apply the community - on ingress or egress.
Comments: text to explain what the community is for. Can use Markdown formatting.
Tags: a list of tags to help identifying and searching for a community.