It's important to level a plywood or wood subfloor before you install any kind of floor covering over it, or bumps and unevenness can create problems. The best material for filling gaps and depressions is patching compound. Fixall is the former brand name of one such product now known as Fix-It-All. It comes in powder form that hardens to a concrete-like consistency soon after you mix it with water. It isn't suitable for leveling a finished wood floor unless you plan to paint it, because it has an unattractive off-white color.
Vacuum the floor before you start, removing wood chips and large dirt particles that will make it harder to spread the leveling compound.
Identify the depressions you need to fill by placing a straight length of 2-by-4-inch lumber on the floor on its edge and noting the places where the floor doesn't touch it. Draw circles around these areas with a pencil.
Pour a quantity of Fix-It-All powder into a 5-gallon bucket. Mix only as much as you can use in about 20 minutes, because it sets quickly and is unusable after it sets. Fill the bottom of the bucket to a level of no more than 4 or 5 inches.
Pour water slowly into the bucket, stirring as you pour, until the mixture becomes a thick, soupy paste. If the consistency is too firm, you won't have time to spread it before it sets. A loose consistency will also help it self-level.
Trowel the mixture from the bucket on the depressions you marked and smooth the surface with the trowel, feathering the edges into the rest of the floor. Work quickly enough to use the entire batch, or you'll have to discard what remains.
Let the compound dry until it lightens into a uniform whitish-gray color. This can take from two to four hours, although it should be stiff enough to walk on after about 45 minutes.
Sand down any high areas of the leveling compound, as well as any bubbles or bumps in the plywood, with a belt sander. Vacuum the sanding dust away when you're done.