What is csing?

Cs stands creep score and this generates most of your gold income. A big flaw in most low elo players is that the only go for kills and ignore the cs. Cs is your most reliable form of gold generation and roughly 26 cs equals one kill(normal bounty, 300 g)

Why should i cs?

As stated above it's the most reliable way to get gold, and almost every role needs to know how to cs. A good way to practice is to go into a custom game and practice csing for 30 minutes. It gets really monotonous but it's definitely worth doing.