Crystallization is when the honey takes on a crystallized, semi-solid state. This is normal, and in fact good—it’s an indication that the honey is pure. Basically, honey is a high-sugar, low-moisture product, so the sugar molecules really want to form crystals, and any pure honey left to its own devices for a while will crystallize. Raw honey tends to crystallize faster because of the higher number of particles (pollen, resin particles, etc.) in it compared to filtered honey.
So people prefer crystallized honey, but if you like the liquid stuff, no worries! Just gently warm the jar by setting it in a warm water bath, no warmer than 104 degrees Fahrenheit (40 degrees C), and the honey will re-liquify. Don’t microwave it as this wouldn’t heat the honey evenly and would risk destroying beneficial enzymes!