The word step has several related meanings:

1. A single movement of lifting your foot and putting it down in a new place. When you take a step forward, you move one foot ahead of the other. A baby's first steps are exciting because they mark the beginning of walking independently. You might also use step to mean the distance covered: “The library is just a few steps away.”
2. One of a series of flat surfaces you climb to get from one level to another. The steps leading up to your school's front door, or the steps on a staircase, let you move between floors. Outdoor steps might be made of stone or concrete, while indoor steps are often wood.
3. One action in a process or sequence. When you follow a recipe, you complete each step in order: first measure the flour, then add the eggs, and so on. Learning long division involves several steps. Missing a step often means you'll need to go back and redo part of the work. Scientists talk about the steps of an experiment. When you're taking things step by step, you're being careful and methodical.

The phrase watch your step means to be careful where you walk, though people also use it to mean “be cautious” in general.