The phrase "jar of jam" simply refers to a container filled with preserved fruit spread called jam.
Sentences with «jar of jam»
When she got through the last jar of jam I bought in less than two weeks I vowed to find a healthier alternative. (myfussyeater.com)
While it was definitely a learning experience, it was also a lot of fun and I'm so proud of my first 10 little jars of jam. (crunchydomesticgoddess.com)
I was foraging my collection of blackberries when a lovely lady said to me I've just made 3 jars of jam with mine.. (thelifesutra.com)