DC means Difficulty Class in DnD 5e; it represents the target number for an ability check or saving throw.
The more difficulty a task is, the higher the DC. When you make an ability check, you roll a d20 and add your ability modifier for whatever is relevant tothe task (determined by the DM).
For example, if you’re examining a magic item, you’d add your Intelligence (Arcana) modifier.
If the result of your ability check is equal to or exceeds the DC of the ability check, you succeed.
The Difficulty Class for a saving throw is determined by the effect that caused it. For example, the DC to resist a spell is equal to 8 + the spellcaster’s spellcasting ability modifier + their proficiency bonus.
The DC of any task is between 5 (very easy) and 30 (nearly impossible), thanks to 5e’s system of bounded accuracy.