It's most likely you're using the client from bitcoin.org.
Synchronizing with the network refers to the download of the blocks in the block chain, which is the transaction history of Bitcoin. The block chain is the record of who spent what and when they spent it, and who owns what now.
Currently, the bitcoin.org client downloads the entire block chain on installation, which is somewhere around 3GB of data. As it downloads each block, it confirms each transaction, which is a disk-intensive activity. Unless you have high spec hardware, expect it to take many hours, possibly even days.
An unreleased version (well, it's released as a beta, but is intended to be a version release 0.8) of the bitcoin.org client alters the syncronization process, which should heavily reduce the time required to get the client functional. If I understand it correctly, the process will be faster through being less disk intensive.