This is eggcrate, it comes in a 2 foot by 4 foot panel and you cut it to fit inside the groove on the top plastic trim of the aquarium. Also cut around pipes or filters, ect. that might stick up above the top. Cut with hand held tin snips or saw.
Most use it to prevent fish from jumping out while allowing air to move thru. Most glass tops do not allow proper air exchange in most cases and can cause heat problems when useing stronger lights.
Make sure lights are high enough above it that it does not melt....never used it under my lights so I dont know how easy it melts. ( Im not cooking it in the pic
and just placed it on my tank to take the photo. )
It will prevent some light from shineing sideways thru it so, your call. Thats what its made for, a diffuser.
The Home Depot near me, keeps it in the grid ceiling section of the store...