You probably use the App Switcher a lot throughout your day-to-day iPhone usage. It's an essential part of multitasking because it lets you easily get back to the apps you've used recently.
On the other hand, it also takes a screenshot of your last activity in the apps you closed recently, so it's a privacy hazard because anyone can peek over your shoulder to see what you've done recently when you're switching between recently-used apps.
ASBlur is a new free jailbreak tweak that helps solve this problem by blurring the contents of your App Switcher app previews.