It means that somebody is at work. It is derived from capitalist societies where it is common for people to be paid a certain amount by the hour (so if you make $5 per hour and work for two hours you make $10). When people are "on the clock" it means that they are being paid to work. This can be used two ways- The first is when someone is on the clock. Employers generally don't want their employees to be socializing or playing games at work and so they may remind their employees that they are on the clock, effectively telling them to get back to work and to play games during their own time. The second is when somebody emphasizes NOT being on the clock. If I am not at work and somebody is asking me to take care of a certain task I may remind him or her that I am not on the clock right now, meaning that I will take care of the task when I am at work, but right now I don't want to be thinking about work related activities since I am not getting paid.