Some companies require from candidates to take skill tests to apply for a job. If a candidate has already taken such skill tests previously, then they can directly apply for the job, otherwise s/he needs to take any missing skill tests before s/he can apply.
To find out if you need to take a skill test, simply go to the job description page, then if you can see the button “Apply for the Job” then you can directly apply. If that button is not available, you will see the list of tests that you need to take before you can apply. A maximum of 3 tests can be required and each test can vary from 20 min. to 60 min. max. Most skill tests are multiple choice questions, and some include a coding question.
You can check the list of assessments to see what each one of them may contain as questions.