Fixed costs are the costs that do not change as the output/volume changes. The cost is set no matter how much you produce or in other words how many units of a produce you produce. A variable cost on the other hand changes based on the output. For example a fixed cost can be the insurance cost. On the other hand, a variable cost is the materials you need to produce a product.
The first will not change no matter how many units you produce (insurance cost will be the same if you produce 10 units or 1,000 units). The materials though will change as you will need a specific amount of raw material per unit of output.