Can something be simple but difficult?
At first glance, that seems contradictory. But if you think on it, you'll realize many, many problems and goals are simple but difficult.
For example, being above-average healthy* is quite simple. You need to eat, exercise, and sleep well. The solution is well known, has only a few key dimensions, and can be explained and understood pretty simply. But it is also extremely difficult to get started on that, break your existing patterns, and do it consistently with delayed gratification for a long time.
Same can be said for developing good relationships, creating a blog or small business, saving money, being at peace, raising a child, etc.
A common mistake people make is conflating difficulty and complexity. For instance, instead of acknowledging that the difficulty in being healthy is in execution or consistency, people may think the fault is in the simplicity. Then they turn the simple solution into something complex, like eccentric diets or sophisticated exercise routines. Now they are hit with a double whammy that's hard to recover from - they lose the simplicity and they get distracted from solving the actual challenge.
Understanding a problem or goal deeply is so key to solving it.
Notes:
[1] * I'm specifically using "above-average healthy" as other forms of healthy like being a pro athlete or fighting a disease or condition is more complex.
[2] Can something be complex but easy? Easy peasy rocket science or thriving startup? Well, that's probably contradictory because complexity is a dimension of difficulty.