Hi,

 

I am new at this platform. I want to know is there any difference between BIOS and manual overclocking? Can anyone give me a detail answer?

Well if you are talking about arm platform then there is no such thing like a bios. makes decision in this case easy.

If you are NOT talking about arm then this is the wrong place for you

 

As @Werner said. ARM boards usually don't use a BIOS. Instead a device tree is used. https://elinux.org/Device_Tree_Reference
 

It is possible to overclock ARM SBCs more than what Armbian provided. But most of them are running close to their highest stable clockspeed.
If you are not experienced enough I would advice you not to do this. You could break a board.

With PCs you can either set the overclock settings in BIOS, or set a setting so you can change these settings in Windows/Linux. I often use this when rendering a video or for gaming.
The difference is that Intel and AMD set their profiles so a computer with a stock heatsink would not overheat. So with better cooling you can often clock a lot higher while being stable.
My I5 2500K has a base clock of 3.3Ghz. While it can run at 4.4Ghz with ease.