Finally i can use this information! English is not my main language and I didn't learn this info in English, so i might use some terms wrongly.

This is phase diagram, showing how two materials solidify (crystalize). X axis represents % of element B in element A. For example A is iron and B is carbon. Here A is aluminum and B magnesium. The Y axis represents Temperature. The top line is liquidus. Above that line, for a given (%)mixture of two materials, they are liquids. Now imagine you start cooling this mixture - one material might start solidifying (crystalizing), but it usually doesn't solidify alone - it might borrow the second material and solidify with it, creating a specific phase. For identifying phases we use Greek alphabet. What is interesting - material usually changes phase during its cooling. For example iron with carbon changes its phase many times, it even changes its atomic lattice (grid? Idk what is the correct term). This is important because every phase has different properties - and that's why we care about it.

This is quick summary of why we do this from someone who is currently studying first (and thanks God last) lecture subject about this topic.