A grove can be an orchard or a clump of trees that doesn't have much undergrowth and occupies a contained area, like an orange grove or a small shady grove of oak trees where you can have a picnic.
You wouldn't call a forest or a large woods a grove — that's too big. And you wouldn't call two or three trees a grove either — that's too small. A grove is somewhere in between. You'll often hear the word refer to a group of trees cultivated by humans, often for commercial purposes, such as an apple grove planted to supply produce to grocery stores.