Every journal publisher has terms and licenses that tells you what you are allowed and not allowed to do with an article that you publish with them. For example, you may not be allowed to send the article to someone else. Always check the terms and licenses so as to make sure that want you intend to do is allowed.
The following are Open Access levels and what they commonly mean. However, do not assume the meaning of a level when a publisher says that articles published are of certain level. Instead, read the terms and licenses to understand what you are allowed and not allowed to do.
Article is allowed to be shared or reused, usually under the condition that the usage is non-commercial.
Author of the article is allowed to self archive. For example, the author is allowed to deposit the article in a repository.
Hybrid is a mix of open access and paid access. It could, for example, refer to a journal where majority of articles are paid access and the author would publish an article open access (usually for an additional fee) in the journal amongst all of the paid access articles. Sometimes it can be double-dipping, which is when a publisher charges an author to publish and then charges for access to the article.
Here are the definitions of OA levels from Taylor and Francis and Springer. Please check the terms and licenses of the academic journal publisher you are publishing with as it may be different.
Definition of Gold and Green (Under 'G') and Hybrid (Under 'H') from Taylor and Francis :
Definition of Gold and Green from Springer: