If something is aimless, it has no clear purpose or goal. An aimless walk is just walking, with nowhere you need to be. An aimless conversation drifts from one thing to another. Said of a person or a life, it is kinder than lazy, but it is not a compliment: it suggests they have not found a reason to point themselves at. It is aim (what you are trying to hit or reach) plus -less (without). You can be without a plan for an afternoon and feel free; you can also feel lost. The word does not tell you which. Though actually, writers sometimes use it as a soft criticism of young people, which is messier than the dictionary’s calm “no purpose”.