The 'tone' of something refers to the manner, feeling, quality or atmosphere. If you 'set the tone' it means to establish the way in which something should be done, for example, whether something is formal or informal. if you were to begin a meeting with a serious statement it would 'set the tone' for the rest of the meeting to be serious.

1. Leaving my wallet in the back of the taxi set the tone for the whole evening. It was a disaster!
2. The lighting and decor in the restaurant set the tone for the dinner, it was magical and romantic.

1. As boss of the company, it's up to me to set the right tone. That's why I never procrastinate or arrive late to the office.
2. Lisa's speech at the conference was highly informative, but also warm and humorous. It set the tone for the whole event.