Proximo is a Heroku Add-on (plugin).
Because the Heroku dyno grid is dynamic in nature, the IP address that a given dyno will be assigned over time will be both dynamic and unpredictable. This dynamic sourcing of outbound traffic can make it difficult to integrate with APIs or make connections through firewalls that require IP-based whitelisting.
Proximo overcomes this limitation by providing a known, static IP address and a tunnel through which your app can send outbound traffic so that it is always sourced from your assigned IP. You can then provide this IP address to an API partner or use it to form the basis of inbound firewall rules to connect to a protected resource such as an internal API or database.