A heterogeneous mixture has a non-uniform composition. When you look at two areas in the mixture you will see something different. This difference might be easily observed with your own eyes, or you might need to look at the mixture with a microscope.
For example, water mixed with dirt is a heterogeneous mixture. If you mix dirt with water, you will be able to see the dirt particles suspended in the water. That means the composition of the mixture is not uniform.
If it was not easy to see the suspended dirt particles, then you would need to look at the mixture with a microscope. If you looked at a dirt particle with a microscope, then you would only see the substances that form these dirt particles. If you looked at the space between the dirt particles with a microscope, you would only see water molecules. This would also tell you that the composition of the mixture is not uniform.
Other examples of heterogeneous mixtures include blood, milk, and smoke. It might not be immediately obvious that these mixtures are not uniform, because when you observe them with your eyes they look uniform. But if you look at them with a microscope it is possible to tell the difference.
Figure 1 - A heterogeneous mixture of water and dirt. The large brown circle represents the dirt particle. Water is represented by the red circle connected to the two smaller circles.