An OpenStack instance is a virtual machine (VM)/VPS created and managed through the OpenStack software platform. These instances behave just like physical computers, allowing you to install and run software and applications.
In the context of cloud computing, an instance is essentially a virtual server. In OpenStack, you can independently create, manage, and delete instances. You have a lot of freedom in your choices: you can create instances with a wide range of specifications, including RAM, vCPUs, and disk space. Additionally, you have the freedom to choose the operating systems you use on an OpenStack instance.
An overview of the instance specifications on TransIP's implementation of OpenStack, the 'Public Cloud', can be found on this page.