The accept_roads parameter controls which types of road are included in the 'snapping' that takes place at the start and end of each journey.
'Snapping' is the process of getting from the starting location to the nearest node on the road network, and then from the nearest node on the road network to the destination location.
There are two possible values for the parameter:
both_drivable_and_walkable(default) - this means that journeys can only start or end on roads that are accessible both by car and on foot. This effectively means journeys cannot start or end on motorways (as they are not accessible on foot).
any_drivable- this means that journeys can start or end on any roads accessibly by car. This effectively means journeys can start or end on motorways.
Other mapping providers (such as Google Maps) typically allow snapping to all roads, including motorways.
Restricting snapping to just roads that are walkable will likely result in more disconnected 'islands' in the driving isochrone, where the gap between junctions on a motorway may be unreachable because it is not possible to snap to the road at that point.