A jelly roll is a synthetic-long-stock in the near month against a synthetic-short-stock in a further month at the same strike. Direction cancels completely. What remains is the cost of carrying stock between the two dates: interest minus expected dividends.
Traders use the quoted roll price to back out the market's implied-dividend and implied-forward for a name, which is often more current than any published estimate.
Example: XYZ at $50. The near synthetic prices at −$0.05 and the six-month synthetic at +$0.55. The roll is worth $0.60, which on $50 over half a year implies about 2.4% annualised net carry. If the risk-free rate is 4.4%, the market is pricing roughly 2% of dividend yield.
Related: conversion, cost-of-carry