bsd.rd is a "RAM Disk" kernel. This file can be very useful; many developers are careful to keep it on the root of their system at all times.
Calling it a "RAM Disk kernel" describes the root filesystem of the kernel -- rather than being a physical drive, the utilities available after the boot of bsd.rd are stored in the kernel, and are run from a RAM-based filesystem. bsd.rd also includes a healthy set of utilities to allow you to do system maintenance and installation.
On some platforms, bsd.rd is actually the preferred installation technique -- you place this kernel on an existing filesystem, boot it, and run the install from it. On most platforms, if you have a running older version of OpenBSD, you can FTP a new version of bsd.rd, reboot from it, and install a new version of OpenBSD without using any removable media at all.
Here is an example of booting bsd.rd on an i386 system:
Using Drive: 0 Partition: 3
reading boot.....
probing: pc0 com0 com1 apm mem[639k 255M a20=on]
disk: fd0 hd0+
>> OpenBSD/i386 BOOT 2.10
boot> boot hd0a:/bsd.rd
. . . normal boot to install . . .
As indicated, you will be brought to the install program, but you can also drop to the shell to do maintenance on your system.
The general rule on booting bsd.rd is to change your boot kernel from /bsd to bsd.rd through whatever means used on your platform.
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