I am using Redhat 9 on my desktop. My computer shut down last night. I went into the BIOS to check on the temperature to find my CPU running at 38c and the system at 50c. It cooled down quickly and I have not noticed any problems today so far. But I do want to monitor the temperature. What can I do to be able to monitor the temperature without having to reboot so I can access the BIOS? I have seen mentioned acpi. How would I get it on my system?

You need two applications: lm-sensors and a client program to display the data.

To set up lm-sensors, follow this guide:
LQ-Wiki: Setting up lm-sensors

Once the sensors are up and running you can choose from a variety of programs to read the data. The most known are Gkrellm and Ksensors. I'm not going to explain how to set them up, there are GUI applications which fairly easy and intuitive interfaces (Gkrellm uses F1 to access the config widget).

BTW: Gkrellm does not need lm-sensors enabled, just that the modules are present, it will load them by itself and display temps, voltages and fan speed w/o need to have the sensor daemon running.