SMB2 is a later version of the old SMB protocol. Both were introduced by Microsoft for local area network (LAN) file sharing. Since SMB showed up in the late 1980s, the SMB family has become the most common LAN file sharing protocol. It's used by most network-attached storage (NAS) devices and file servers. SMB2, which was introduced in Windows Vista in 2007, is now supported by Samba and Linux and, so Apple says, by Mac OS X.
Alas, SMB2 is supported by Mavericks more in theory than in practice. There have been numerous user reports of not being able to connect at all with their NAS and file servers. Others are reporting that they can't get iWorks to write to network drives using SMB2.
Fortunately, there are ways to mitigate this problem. First, and both crudest and simplest, is to force your Mavericks Mac to use SMB instead of SMB2.
To do this you'll need to create a nsmb.conf file in your home directory with the path ~/Library/Preferences/nsmb.conf. There are several ways to do this. First, you can up a terminal from Utilities and paste in the following line from the shell prompt:
The above must be entered as a single line with a return at the end. You can also create this configuration file with a text editor such as vi from the terminal. Do not, however, try to create this file with a word processor. You'll end up with formatting characters in the file that will ruin it for configuration purposes.
However, none of these may work depending on whether your file server or NAS supports SMB or CIFS. While they're almost certain to support them, their default settings may not support these older protocols. In that case, you — or your network administrator — will need to set them to support SMB and/or CIFS.
The best thing you could do — which is probably too late if you're already reading this — is to hold off "upgrading" to Mavericks. It seems to me to be one good update away from being ready for prime time.