Your client needs to catch up with the blockchain and load all the blocks in existence to your harddrive.

This means 156505 blocks or 768 MB data (see http://allchains.info/). This may take a while.

Every time you start your client after this, it will download all blocks, that were created since the last run.
While this is shorter, I experienced a "waiting time" of about 1 minute for every day of blocks created ...