Pre-conditioner is a weak solution of finish(shellac is my preferrence) that soaks into the softer areas of the grain of a wood. This prevents stain from concentrating in those areas---which is why pine/cherry blotch. I have tried several off the shelf preconditioners and did not like any of them. One really darkened the wood I was using. The kind I have found works well is a one pound cut of blond(white) shellac. That is made two ways. One pound of dewaxed shellac flakes is disssolved in one gallon of denatured alcohol. Normal shellac concentration for finifhing is three pounds of flakes in one gallon.

Or, for most DIYers, use the quart can off the shelf(shellac shelf life varies by manufacturer from 18 to 36 months) and mix a bit more than 2 parts alcohol to one part shellac.

Sanding sealer is simply that---a product to create a smooth surface after sanding. Using a sealer may be less work than applting several coats of finish , sanding after each coat, until the desired surface is reached.