 <p>Those are <a href="https://gcc.gnu.org/onlinedocs/cpp/Macros.html" rel="nofollow">macros</a> declarations.</p> <p>Whenever you have <code>lowByte(0x1234)</code> in your code, it will be replaced with the right part of the macro, substituting arguments with their values, that is <code>((uint8_t) ((0x1234) &amp; 0xff))</code>.</p> <p>This step is performed by the <a href="https://en.wikipedia.org/wiki/C_preprocessor" rel="nofollow">preprocessor</a> prior to compilation.</p>