means A thick, sweet spread made by boiling fruit with sugar until it sets into a sticky, jar-ready preserve.

from From the verb 'jam,' meaning to press or squeeze tightly together — which is exactly what you do to the fruit. The word likely echoes the sound and sense of cramming something into a tight space, and by the early 18th century it had stuck to the crushed-fruit conserve. The same root that lets you jam your foot in a door lets you jam strawberries into a jar.