What does Overclock (OC) means?

Overclocking is the process of running a computer component at a higher clock rate (more clock cycles per second) than it was designed for or was specified by the manufacturer, usually practiced by enthusiasts seeking an increase in the performance of their computers. Some purchase low-end computer components which they then overclock to higher clock rates, or overclock high-end components to attain levels of performance beyond the specified values. Others overclock outdated components to keep pace with new system requirements, rather than purchasing new hardware.[1]

People who overclock their components mainly focus their efforts on processors, video cards, motherboard chipsets, and random-access memory (RAM). It is done through manipulating the CPU multiplier and the motherboard's front side bus (FSB) clock rate until a maximum stable operating frequency is reached, although with the introduction of Intel's new X58 chipset and the Core i7 processor, the front side bus has been replaced with the QPI (Quick Path Interconnect); often this is called the Baseclock (BCLK). While the idea is simple, variation in the electrical and physical characteristics of computing systems complicates the process. CPU multipliers, bus dividers, voltages, thermal loads, cooling techniques and several other factors such as individual semiconductor clock and thermal tolerances can affect it. Every component has its unique limits with higher clocks and voltage. Two different CPUs may clock completely differently, one being stable with higher clocks at a similar temperature as the other. The bus and memory stability and tolerances also affect how stable an overclocking can be.[2]
Source Wiki

For more detailed info refer to the link below
http://www.webopedia.com/DidYouKnow/Comput...verclocking.asp

The tutorial on how to OC is in the spoiler below

DO THIS AT YOUR OWN RISK . IT MIGHT DAMAGE YOUR PHONE AND WE DO NOT TAKE RESPONSIBILITY IF ANYTHING HAPPEN TO YOUR PHONE

Step 1 First Connect your N900 to your computer with cable via mass stroage
Step 2 Second decide which speed do you want to OC and download the file below

700mhz Consider as safe
750mhz
800mhz ( Most ppl flash this speed)
825mhz
900mhz (Risky)

Step 3 Copy the above file you just download to the root directory you just download aka (Immediately u enter your internal memory (32GB) thats the place contains .music, docs, .videos etc etc paste your file there)

Step 4 Install rootsh to access to root later http://maemo.org/downloads/product/Maemo5/rootsh/ or find it from app manager

Step 5 Now on your nokia N900 go to MENU > X Terminal > Open it

Inside X terminal ( X terminal is case sensitive )
Step 6 Type root or sudo gainroot > Enter

Step 7 Type fiasco-image-update /home/user/MyDocs/zImagexxx.fiasco > enter
XXX indicates the speed file u download example (Type fiasco-image-update /home/user/MyDocs/zImage900.fiasco) i choose 900mhz file

Step 8 *IMPORTANT* Restart your phone and access conky to confirm your success of OC

To restore back to original speed

Step 1 In your nokia n900 Menu go to More... > X Terminal
Step 2 Type Root > Enter
Step 3 Type apt-get --reinstall install kernel kernel-flasher > Enter
Step 4 That is all
Source http://maemocentral.com/2010/04/04/how-to-...the-nokia-n900/