By default, a variable's value can only be used in the topic where this variable gets created. However, you might want the bot to use the same value across topics. This means the bot can remember the necessary context when a conversation spans multiple topics. In some systems, these types of variables are known as global variables. In Power Virtual Agents, these variables are called bot variables, because they apply across the entire bot.
Reference:
https://docs.microsoft.com/en-us/power-virtual-agents/authoring-variables-bot