D&D 5e introduced the concept of advantage and disadvantage. If you have advantage on an ability check, attack roll, or savings throw you roll d20 twice and use the higher roll. For disadvantage, you take the lower roll.
How beneficial is advantage? Very.
For a task of medium difficulty (DC 15), there is roughly a 50% chance of success with advantage. Without advantage, there is only a 6/20 or 30% chance of success. Since a 50% chance of success is equivalent to DC 11, effectively, advantage at DC 15 is equivalent to a +4 bonus.
Here is how the statistics break down for each difficulty class (DC).^{1}
DC  Success (%)  Effective Bonus 

1  100  0 
2  99.8  +1 
3  99.0  +2 
4  98  +3 
5  96  +3 
6  94  +4 
7  91  +4 
8  88  +5 
9  84  +5 
10  80  +5 
11  75  +5 
12  70  +5 
13  64  +5 
14  58  +5 
15  51  +4 
16  44  +4 
17  36  +3 
18  28  +3 
19  19  +2 
20  10  +1 
Since a typical character already has a bonus (e.g., +3) to the roll, the situation is even more advantageous (ha!). With a +3 bonus the player needs to roll 12 or above to succeed against DC 15, so the chance of success is about 70%. Compare that to 45% without advantage.
In conclusion, granting advantage provides a significant bonus to the player — equivalent to a +4–5 bonus for typical DCs.

The Python script advantage.py was used to generate these results. ↩