A mix of herbs, bound together and cooked. The nearest modern equivalent is something like Dock Pudding.

Original Receipt in 'The Forme of Cury' by the Chief Master-Cook of King Richard II, c1390 (Cury 1390)

ERBOLATES
Take parsley, mint, savory, sage, tansey, vervain, clary, rue, ditayn (?), fennel, southernwood. Hew them & grinde them small, mix them up with eggs. Butter a cloth & do þe forcemeat thereto & bake it & present it forth.