You would never be allowed drive without first doing an exam or get a job without first doing some sort of an interview right?
So the same can be said for your students - you need to be able to gauge or ask them to demonstrate their understanding, acquired knowledge or skills of a particular outcome or unit of learning.
An assessment of student learning is asking you - how do you know that they know? What can you do with your students to evaluate this and do you need to record evidence of it.
An assessment does not have to be summative (AoL/Assessment of Learning) like a test or exam, it generally is more formative (AfL/Assssment for Learning) for example it could be a question and answer session, review of a case study or getting a group of students to create a mind map.
So in Business Studies for example you might have questioning, discussion around a case study and you observing and questioning during a walking debate or students presenting within a group of promotional material and write a letter to relevant consumer agency or letter of complaint. You could use Quizlet to send a bank of questions to students (which you can post in your class group) and automatically collate the answers.
An assessment does not have to be a formal data point, exam or test, it is simply a way for you to confidently know or gauge your students capacity of understanding, knowledge or skills of an outcome or unit.