It’s the sentry ward. When you load into a match, go to the vision tab in the ITEM SHOP. It’s next to the “All item” tab. Then buy it, it’s actually free but it allows you to find enemy wards so they lose vision on the map. Typically jungles buy it so they can de-ward enemy wards.
Adc stands for attack damage carry. Their job is to get as many kills as possible and do the most damage in the game (usually physical dmg). They are usually squishy and die easily. They need to get ahead early game by killing heroes and last hitting minions. Then use position to stay on the edge of team fights dealing massive damage.