When you enter a command at the shell prompt, your shell forks a child process in which to execute the command.
The parent process (your shell) waits for the command to complete and for its child process to die. Until then you are unable to enter another command. Commands entered in this way are known as foreground processes.
Once the command completes and the child process dies, the parent process is reactivated, the shellprompt returns, and you can enter another command.