This represents a permission from the US Government to work legally in the US as a Support Staff or Counselor at a Summer Camp. It runs from the start date of your camp contract, until 30 days after the end date of your contract. This means that you have 30 days in which to travel within the USA before you are required to leave the country.
The visa itself is attached inside your passport. It shows your photo and other information. You may only work as a Counselor, or as Support Staff at the camp for which you have a contract. You must arrive at your camp on the date designated on your camp contract, or your visa may be cancelled.
Camps are required to notify the US Department of State that you have arrived. If you do not, then your presence in the USA becomes illegal. So just double check that they do.