Expression used to describe something that is very easy or simple to do. The idea is that taking a walk in a park is generally a leisurely and uncomplicated activity, and this imagery is used metaphorically to describe other tasks or challenges that are similarly straightforward.
It fits into a long tradition of using everyday experiences and imagery to create idioms that describe more abstract concepts or experiences. The phrase is quite common in English-speaking countries and is easily understood to mean that something requires little effort. So, if someone says that a task was "like a walk in the park," they mean that it was not difficult or challenging for them. Similarly, if you're about to undertake something new and someone tells you it's going to be "like a walk in the park," they're trying to reassure you that you should not anticipate significant difficulties.