Humans have this exceptional capability of learning. Ever thought about how humans learn a skill?
Humans learn by observations, and then putting these observations under practice, more practice makes humans more good at that skill. Based on practice, some new observations are unveiled. These new observations lead to new learnings. In this way, we humans update our understandings and get better at a skill with time. This is the idea of self-learning.
On the other hand, computers are explicitly programmed to do a task. Based on instructions, computers do that task. No matter how many times a computer repeats that task, it will perform exactly how it performed in the first run. What if computers can also be like humans and evolve with more and more practice and can self-learn. So this is what machine learning aims to do. To make machines learn without being explicitly programmed.