What does it mean 2.1 to 2.9 Ghz

Solution
"Ghz" is a measurement of operations per second. It's about speed, not memory.
I assume you're referring to a CPU? Most modern processors have a range of frequencies (operations per second) at which they can run. Clocking up higher (=faster = more operations per second) requires more power and produces more heat. In lots of situations (particularly laptops or mobile, but even in desktops too) CPUs are heat constrained. In other words if they run at their highest speed they'll generate too much heat and have to slow themselves down to avoid getting to hot. In some situations you end up power constrained (it could run faster but the CPU is drawing more than it's rated power).

To try and balance performance and heat/power, modern...
"Ghz" is a measurement of operations per second. It's about speed, not memory.
I assume you're referring to a CPU? Most modern processors have a range of frequencies (operations per second) at which they can run. Clocking up higher (=faster = more operations per second) requires more power and produces more heat. In lots of situations (particularly laptops or mobile, but even in desktops too) CPUs are heat constrained. In other words if they run at their highest speed they'll generate too much heat and have to slow themselves down to avoid getting to hot. In some situations you end up power constrained (it could run faster but the CPU is drawing more than it's rated power).

To try and balance performance and heat/power, modern CPUs have a base frequency and a boost or turbo frequency. When hit with a significant workload they can usually boost/turbo up to the higher frequency for a short time (which is sometimes enough to finish the workload anyway), but if the workload continues, they usually have to reduce the frequency to stay within power or thermal limits. The base frequency (the lower one) gives you an indication of the minimum frequency the CPU should be able to maintain if it's fully taxed for a significant period of time.
 
Solution
It's worth noting as well that all CPUs now have multiple cores (that is effectively 2-8 CPU cores within the one physical processor). Lots of software is not actually able to use more than 1 or 2 cores. Quad core CPUs are pretty common, yet their often given a workload that can stress only 1 or 2 cores, leaving the remaining cores idle with nothing to do. In that case the processor allows the cores that are being used to use the higher boost/turbo frequencies, generating more heat, yet remaining within thermal constraints as the remaining cores are idle. Those workloads may have 1 or 2 cores running at the higher 2.9Ghz.

When the CPU is given a workload that can stress all 4 cores (or however many cores you actually have), it'll quickly have to clock them lower, towards the 2.1Ghz, to keep temperatures/power at acceptable levels.
 

aakarshan

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It is called turboboost normally used by processors to increase there frequency.
Normally processors runs at their base frequency 2.1 GHz in your case and boost up to higher frequency when the performance is needed 2.9 GHz in your case.
 
It is the range at which a component can run, I assume it is the cpu. When you don't need much processing power then the cpu should run at 2.1ghz. When you are doing something demanding like video editing which requires the cpu then it will increase its speed to 2.9ghz if it need it be. The time it stays there will depend on the build of the laptop or desktop.