Simple Cpumeter is a simple system monitoring tool. It runs on the menubar and it displays the current CPU load.
This basically means that the app tells you how busy your CPU is. There are two ways in which the application can show you this information. The first, and default one is via a green bar. When the bar is completely green, your CPU is at 100%. When it is black, the CPU is idle. The application scans your CPU usage at regular intervals. The default update time is 0.5 seconds, but this can be changed to 0.1, 0.2, or 1 seconds. You can access the application settings by clicking on the menubar icon. From here, you can also change the display mode. The default one is the icon I described above, but the CPU load can also be expressed in percentages. I find this more useful than the green bar because the reading is more precise. From the menu, you can also launch the Activity Monitor, which is the system utility that lets you fully monitor your Mac.
In short, this application is free and it works. However, there are other free apps that let you not only monitor the state of your CPU, but of your entire system, with better features and more options.