Solid Hardwood, Engineered and Laminate Flooring - Leveling a sub-floor

I know I've seen the answer to this question before on here but I've been searching for awhile now and can't find it. Would someone be able to pass along the answer to me again to the following...

I will be installing a Pergo laminate with the attached underlayment over a plywood floor. I know I can sand down any high spots on the plywood but what do I use to fill-in any low spots including any little open joints between the plywood?

I thought I remembered hearing something about a leveling compound and/or some type of felt but I'm not sure what brand or specific type to buy.

Thank you!

Roofing felt, cardboard, shingles,

I wouldn't worry about gaps smaller then an inch, the flooring will bridge across that, as long as both sides are even with each other.

Thank you for your insight! That eases my mind about what to do. Thank you again!