Mevushal wine means that the wine has been cooked, according to R' Moshe Feinstein ztz"l above 180F. Pasturization usually achieves this, so today that almost all wines and grape juices are pasturized they all have the status of mevushal according to R' Moshe and most US hashgachos.
Since the bishul process degrades the quality and flavor of the wine, it renders it unfit to be brought as a korban on the mizbeach and also makes it unlikely that an idol worshipper would pour it as a libation to his diety. Therefore, the issur of yayin akum does not apply to wine that is mevushal. However for this same reason, it is preferable to use non-mevushal wine for the 4 cups and some say also for kiddush. Today pasturization is done in a way that does not significantly affect the flavor of the wine, and many Kosher mevushal wines have won awards at international contests. Based on this R' Elyashiv shlita rules that pasturization does not render a wine mevushal. But as I mentioned previously most hechsherim in the US rely on R' Moshe.