Its a polyurethane foam cell- it prevents the fuel from being sloshed around. Most boat tanks and race car fuel cells have them. There is a pickup just as a regular tank has normally with a sump built into the tank and fuel lines coming out of the bottom. (hence the ease, of getting low fuel out, and no starvation).
Some have them come out of the top, just with a sump for the pickup in the base...
It doesnt nec absorb the fuel, it just prevents it from sloshing around.