What is meant by client side script?

It is the script which is downloaded and run on the client side. That is, it is downloaded from the server and then run on the client computer .The client is the person who uses the web application.

For example: when you download from rapid share using free user account, you will have to wait for 30 seconds. The script is downloaded from server and run in your system. After 30 seconds, the script calls the server back for the download.