Elevation gain is the total amount of uphill climbing on a course, measured by adding up every rise from start to finish. It's usually given in metres and is a better indicator of course difficulty than net elevation change alone. A hilly marathon can have a low net elevation change but still demand a lot from your legs.
It's the sum of every uphill section on the route, added together regardless of the downhills in between. A course with 300 metres of elevation gain has climbed that much in total across the race.
Net elevation is simply the difference between start and finish height. Elevation gain counts every climb along the way, so a rolling course can have high gain even if start and finish sit at the same height.
There's no fixed cutoff, but anything above a few hundred metres of total gain is generally felt as noticeably hilly. Course profiles published by the race organizer are the best way to judge a specific route.
Most race websites publish an elevation profile chart alongside the course map. GPS watch apps like Strava also calculate it after you upload a run on that course.
Plan to slow down on climbs rather than fighting to hold flat-course pace, and use downhills to recover, not sprint. Training on similar hills beforehand also makes a real difference on race day.