Moderate compression is the sweet spot for most cyclists and is the most common level you'll find in cycling pants. It provides noticeable muscle support that helps reduce vibration and fatigue on rides, but it doesn't feel restrictive or tight like firmer options can. You'll stay comfortable for multi-hour rides without that "squeezed" sensation.
This level works well for general road cycling, training, club rides, and most everyday riding conditions. If you're new to cycling-specific pants, moderate is the best place to start — it balances support and comfort so you can focus on enjoying your ride rather than adjusting your kit.
Firm compression does offer more muscle support and can improve blood flow, but it takes getting used to and is really designed for racing or high-intensity efforts. For most riders, moderate compression delivers the benefits without the discomfort, and you can always upgrade later if you find you want more support.