In the below link i have covered the below points:

Visit the link to get the understanding of onload client script-

https://www.youtube.com/watch?v=xjtkfH0KTH8&feature=youtu.be

1. What is Client Script

2. What is onload Client Script

3. Scripting is done for the fields such as remove option from choice list, make field readonly, set value in a field and set field as mandatory etc.