Cucumber is a testing tool that supports Behavior Driven Development (BDD). It offers a way to write tests that anybody can understand, regardless of their technical knowledge. In BDD, users (business analysts, product owners) first write scenarios or acceptance tests that describe the behavior of the system from the customer's perspective, for review and sign-off by the product owners before developers write their codes. Cucumber framework uses Ruby programming.
In Cucumber Behavior Driven Development, whatever you write must go into Given-When-Then steps.
Example
Given that a library is added to simply And I am logged in
When I shall press get Or I press borrow Or I press reserve And I have internet connection And My account has no fines And My account has not expired
Then Book is on my bookshelf
Gherkin is the a defined syntax for the above test scenario. It is a set of special keywords to give structure and meaning to executable spec
More can be learned here https://cucumber.io/docs/gherkin/
