cAdvisor is part of the kubelet, which runs on every node. It collects information about the CPU/cores' usage, memory, network, and filesystems of each container. It provides a basic UI on port 4194, but, most importantly for Heapster, it provides all this information through the Kubelet. Heapster records the information collected by cAdvisor on each node and stores it in its backend for analysis and visualization.
The cAdvisor UI is useful if you want to quickly verify that a particular node is set up correctly, for example, while creating a new cluster when Heapster is not hooked up yet.
Here is what it looks like:
