The phrase "brilliant header" refers to a very skillful and impressive move in the sport of soccer, where a player uses their head to strike the ball in a very skilled and effective way. Full definition
Five minutes later Davinson Sanchez was lucky not to be carded as well after fouling Alexis, but was rightly punished when Ozil's brilliant free kick was met by an equally brilliant header from Mustafi to put us 1 - 0 up, and the crowd goes crazy! (justarsenal.com)
Theo on the other hand has often taken up that empty space, hence his brilliant header after a lovely ball from Alexis, and his work rate, tackling back and general intensity have been spot on. (justarsenal.com)