The sample space is the set of all possible outcomes of an experiment. When you flip a coin, the sample space is {Heads, Tails}. When you roll a die, it is {1, 2, 3, 4, 5, 6}.
Each outcome in the sample space is distinct. No overlap. If you roll two dice, the sample space has 36 outcomes: (1,1), (1,2), ..., (6,6).
Listing the sample space is the first step in calculating probabilities. Once you know all outcomes, you can count how many satisfy a condition.