On Ball and Kerr branded jars (which are made by the same mason jar manufacturer), the freeze-fill line is a thin line found just below the threads, about an inch from the rim.
The purpose of the freeze-fill line is to indicate the maximum amount of liquid you can safely store in your jar without risking a cracked jar when the contents freeze and expand.
Jars that have shoulders, such as regular- and wide-mouth quarts, are not suitable for freezing liquids unless you fill them to no more than 1 inch BELOW the shoulders.
I’ve successfully frozen liquids in quart jars by making sure I leave plenty of headspace (about 3 inches) for the liquids to expand upward.
If you aren’t comfortable with this, but like the convenience of jars and need to store more than jelly jars allow, you can also try plastic freezer jars. (These are great, and recent versions have better fitting lids than the previous Ball-branded versions.)