ELO is a rating system originally developed for chess that measures the relative skill level of teams. In college football, it provides a more accurate ranking than traditional polls by empirically considering strength of schedule.
Teams start each season with ELO ratings based on their previous year's final ELO ratings, regressed 5% towards the mean of 1500. New teams or teams without history start at 1500.
Ten rating points are one point of spread on a neutral field: a team rated 30 points higher is a 3-point favourite, and playing at home is worth another 3.3. The ratings include our adjustment for how each team has been running against the betting market, so the gap is the spread the model actually predicts.