A one-dimensional symmetry group is like a group of friends all standing in a line, holding hands. This line can be as long or as short as we want it to be.
Now, imagine that each friend in the line is wearing a matching t-shirt with a picture on it. The picture is the same on each t-shirt, and it looks exactly the same no matter which friend you are looking at.
This group of friends is a one-dimensional symmetry group, because if we flip the line around (like turning it upside down), the picture on each t-shirt would still look the same. It doesn't matter which way we look at the line, the pattern stays the same.
In the world of math, we can use this idea of symmetry groups to help us understand patterns that look the same no matter how we move them. And a one-dimensional symmetry group is just one example of many different types of symmetry groups.