Selenium and cucumber are completely two different things in terms of functionalities / usage.
Selenium is a library / tool which you use for automating the manually performed actions on browsers. Currently its the most preferred library for ui automation of browsers.
Cucumber is something which enables you with behaviour driven development(BDD). BDD is writing the scenarios in the language you like for eg., English so that it gives you more readability [Usually we call it as Gerkhin language which uses keywords like Given, When, Then etc.,)
An example cucumber scenario will be something like :
Given I am in gmail homepage
When I enter the user name
And I enter the password
Then I will be successfully logged in
Combining these two will provide you tons of advantages like providing readability to your code so that any one in your team say PM, PO, Developer, Tester everyone can get an idea of what your code will do & what are the scenarios you have covered & reuse the steps easily where ever required.
We will be having the underlying implementation of these steps in your code
