As illustrated, you are likely seeing delamination of the screen backlight diffuser - this typically resulting in inconsistent illumination or patchy bright regions of the screen. The only remedy is replacement of the iPad screen assembly.
Be aware that the screen is the single most expensive component of your iPad. Unless you have benefit of an active AppleCare+ plan, an out of warranty repair will be expensive.
Before committing to a screen repair, you might be well advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to hands-on test and assess your iPad. If a defect is identified, you'll be in the best possible place to explore your available options:
Genius Bar Reservation and Apple Support Options - Apple
For practical purposes, in lieu of repairing and returning your own iPad, Apple usually exchange a qualifying iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair. Unless you have benefit of an active AppleCare plan, an out-of-warranty repair while expensive will be somewhat less than the cost of a replacement iPad.
You can verify eligibility and cost for your region/country here:
iPad Repair & Service - Apple Support
If you have an active AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.