Initial Jobless Claims is a report issued by the U.S. Department of Labor on a weekly basis. The employment situation is extremely important for a macroeconomic analysis, so the financial markets track employment indicators, although this is a low impact indicator compared with the monthly BLS's "Employment Report". This report tracks how many new people have filed for unemployment benefits in the previous week. It is a good gauge of the U.S. job market. For instance, when more people file for unemployment benefits, fewer people have jobs, and vice versa. Investors can use this report to gather pertinent information about the economy, but it's a very volatile data, so the four week average of jobless claims is monitored.
The report is released at 08:30 EDT on Thursdays and can be a market moving event.
Initial jobless claims measure emerging unemployment, and it is released after one week, but continued claims data measure the number of persons claiming unemployment benefits, and it is released one week later than the initial claims, that's the reason why initial have a higher impact in the financial markets.