While a process is running, what if it wants to block (or mask) certain signals? This is indeed possible via the API interface; in fact, the second member of the sigaction(2) structure is the signal mask, the mask of signals to block from delivery to the process while the signal handler function is running. A mask typically implies a bitwise-or of signals:

...
/* Additional set of signals to be blocked. */
    __sigset_t sa_mask;
...

Do notice the previous comment; it implies some signal is already being blocked. Yes, indeed; let's say a process traps a signal n via the sigaction system call. At some later point that signal n is delivered to it; while our process handles the signal—that is, runs the code of its signal handler—that signal n is blocked from delivery to the process. For how long is it blocked? Until we return from the signal handler. In other words, the OS auto-blocks the signal currently being handled. This usually is precisely what we want, and it works to our advantage.