With its single proton and single electron, hydrogen is the most abundant, simple and well-understood atom in the Universe. Its spectrum has been measured to very high precision. Antihydrogen atoms, on the other hand are poorly understood. Because the Universe appears to consist entirely of matter, the constituents of antihydrogen atoms -- antiprotons and positrons -- have to be produced and assembled into atoms before the antihydrogen spectrum can be measured. It's a painstaking process, but well worth the effort since any measurable difference between the spectra of hydrogen and antihydrogen would break basic principles of physics and possibly help understand the puzzle of the matter-antimatter imbalance in the Universe.
ALPHA is an experiment at CERN’s Antiproton Decelerator facility, able to produce antihydrogen atoms and hold them in a specially-designed magnetic trap, manipulating antiatoms a few at a time. Trapping antihydrogen atoms allows them to be studied using lasers or other radiation sources.