Service Engine Soon - I have the code

I have a 2003 trailblazer, 66,000 miles.
The service engine soon light is on and I get the following code:
P0128B - Engine Coolant Tempreture below Thermastat regulating tempreture.

What does this mean needs to be fixed?

Welcome to AF.
It is a good indication that the thermostat is not working right or not holding the engine tempt up tp the 195 degree range.
Is the engine running cold or is it getting up to normal operating tempt.

Is the coolant system full of coolant?
I would change the thermostat and service coolant.
Clear code and see what it does.
Any other codes?
The code can also be set if the long term fuel trim is over 25 %
MT

I have a 2003 trailblazer, 66,000 miles.
The service engine soon light is on and I get the following code:
P0128B - Engine Coolant Tempreture below Thermastat regulating tempreture.

What does this mean needs to be fixed?

Had this code many time and found that the customers were taking out the
thermostat thinking they did not need them here in the tropics DR

Thanks for the replies,
The coolant level is good. I'll try changing the T-Stat, clear the code and see what it does.
The SES light turned off today while driving to work. Is it normal for it to cycle off after a while?

Yes.
When the pcm does not see A PROBLEM OR it is not acting up the light will go out or the computer will clear the code.

MT