The secure bootloader is the first observable code that runs on the iPhone and iPod Touch. Its main purpose is to start LLB. It has a secondary purpose as an image verification “library” for iBoot/iBEC/iBSS, and contains the only known code on the systems to make use of the PKE (Public Key Encryption) - the RSA accelerator. It's also responsible for (some of?) the handling of the DFU (Device Firmare Upgrade) mode.
It resides in the so-called VROM, which is typically mapped to the 0x20000000 region in virtual memory. The 'V' probably stands for 'virtual,' and likely alludes to the fact that it is copied to memory before being used. It is not directly mapped ROM as there is code in iBEC that patches the VROM. However, it is still uncertain whether the backing store for this code is truly read-only, or like in the baseband, the loader is writable but rarely touched.