A Grand Cross is a planetary alignment in the astrology chart. It consists of four planets at 90-degree angles with each other and each being in a different astrological quality. Every sign of the zodiac is either cardinal, mutable, or fixed. Therefore, a Fixed Grand Cross happens when all four planets involved are in the mutable signs of fixed signs of Taurus, Leo, Scorpio, and Aquarius.
This brings a very different energy to the Grand Cross than say a Cardinal Grand Cross or a Mutable Grand Cross, for the fixed energy is probably the most stubborn and challenging of the three, even though Grand Crosses can be challenging enough in and of themselves.