All woods have a natural texture resulting from the size and distribition of the woods pores. This texture has the largest overall effect on the appearance of wood. (Think of a piece of maple vs oak, the color is similar but they look different). By filling these pores to different extents you can significantly change the appearance of a wood.

Sealers: Used to penetrate into the pores of lumber these will cure and stop finish from flowing into them. Every first coat of finish is a sealer. Separate, dedicated sealers exist that are easier to sand than certain finishes, making initial surface perfection easier.

Filling the pores with finish: Lay on a few coats and allow it to cure. Cut it back with your final sandpaper grit (probably 220, maybe 320 if you are using a finish that melts into itself). This is possible with any finish that cures hard (so no oil, wax, oil varnish blends). If you still have minor imperfections just add another coat.

The problem with this is it can be very slow, if you are working with something tight grained like maple or cherry it might work in one coat, but if you are working with a larger pored wood such as oak or walnut you will be better off with a dedicated pore filler.

Paste wood filler: This is a binder (finish) with finely ground silica added for body. You can add pigments if you want the grain to pop. These are usually oil or varnish based, I prefer water based because the dry time is much faster.

Egg white: This is my go to pore filler, it dries quickly, can be pigmented, is super cheap, sands and scrapes easily and I almost always have eggs around. Like other fillers, and wood itself this isn’t water proof; but your top coat will absorb into them and give protection.

Just separate the white from the yolk, discard the yolk and mix the white with some sanding dust from your project, and rub them on to the surface (don't get them frothy) with your fingers or a fine cloth, let it dry and sand off.

Remember when you are filling pores: The smoother your fill job the less sanding there will be to do! First: Block sanding to flatten a filler coat. Second: Unfilled vs Filled. Third: These drawings represent how layers of finish will react with the wood grain on an unfilled vs a filled piece.