Flat or unarticulated knee pads have padding with no pre-shaped bend, so they simply wrap around your knee when you strap them on. They're typically the most affordable option and work fine for casual, low-intensity skiing where you're mostly cruising and not spending much time in a deep knee bend.
The downside is that flat pads tend to bunch up or gap when you flex your knee, which can get uncomfortable over a full day on the slopes. That bunching behind the knee is especially noticeable when you're skiing aggressively or tucking. If you're doing any high-performance skiing, racing, or freeskiing where you're constantly bending and extending, you'll likely find flat pads frustrating.
Save flat pads for mellow resort days or if you're just getting started and want basic protection on a budget. For most skiers, spending a little more on pre-curved pads is worth it for the comfort alone.