Combination stones are great for a beginner sharpener, starting out on a tight budget. As the name implies, they're a combination of two different grits, one on either side. The rougher grit is used to establish the edge, whereas the smoother grit is used to polish and refine it.
As they wear, combination stones can become uneven and rock as you use them. Grab a truing stone to keep your stone flat and avoid this issue.