IPA is Isopropyl Alcohol..basically removes any oils/residues off the paint/wheels etc so you know you are sealing whatever surface you want to protect, rather than sealing a layer of polish residue which will probably come off the next time you wash the car, then blame the sealant for being crap lol

I love using Gtechniq C5 as suggested above, or the cheaper CQuartz dLux I think its called which is currently on my Merc wheels!