Synthrapol is a type of liquid soap. It is sold in quilt shops that cater to dyers and is also widely available online from Amazon, Dharma Trading, etc. It requires hot water to be effective. If you use it in cold water, then I would make sure the washing machine uses a *lot* of water so any dye bleeds become very diluted. Many domestic front-loading washers do not use enough water; laundromat front-loaders and domestic top loaders typically use enough water.
In my opinion, Synthrapol is more effective than color catchers (although you can use both). Color catchers pick up dye particles, but there is no guarantee they will pick up all of the loose dye particles. If you have a fabric that bleeds excessively and you don't use a lot of water, the color catchers will not be enough. Synthrapol works on preventing loose dye particles from settling back into fabric. If you have something that already has a dye bleed in it, washing repeatedly with Synthrapol in hot water will usually get rid of it. (This can occasionally save a quilt.)
Again, in my opinion, the most important thing is to use *lots* of water so any dye bleed becomes very diluted. Adding Synthrapol decreases the chance of a dye bleed even more. Color catchers are fine for small bleeds but will be overwhelmed by large bleeds.