As the concentration of potassium inside guard cells increases, water enters the cell by osmosis. But why does the swollen causes the stoma to open? Does it relate to the thickness of the cell wall? If so, how?
The link that @tyersome mentioned in the comment does a great job of answering your question. In the event that you read it and still have some confusion, here is a metaphor: imagine guard cells as two modeling balloons (the long kind that clowns use to make figurines) that are slightly inflated and attached together at both ends. Since they are only slightly inflated, they are able to sit flush next to each other. Now let's say you inflate the balloons. Since they are still attached to each other at both ends inflating them will cause them to bow outward and away from each other. The expansion will cause there to be space between the two balloons.
This is not a perfect analogy since guard cells have a varied amount of rigidity in their cell walls contributing to the shape when water enters. But maybe this can help you visualize the concepts introduced in the link above intuitively.