Depending on your permissions, you can create Terms, Term Sets and Term Groups on Site level or on Tenant level. However, if you created them on site level and want to reuse them on another site, there are some additional steps required. The article by SharePoint Maven explains how to achieve this.
In SharePoint, the term store is a centralized repository for storing and managing terms that are used in a SharePoint farm / tenant. It is used to store terms and their associated metadata, such as descriptions and synonyms, in a hierarchical structure called a term set.
The term store is useful for managing terms that are used across multiple sites in a SharePoint, as it allows users to create, edit, and manage terms in a single location. This can help ensure that terms are used consistently across the tenant and that users can easily find and use the terms that they need.
In addition to storing and managing terms, the term store can also be used to define relationships between terms, create custom metadata for terms, and assign terms to specific content within SharePoint sites. This can help users more easily find and classify content, as well as ensure that content is consistent and accurately labeled.