As your magic becomes stronger, you are having more and more difficulties containing it to the effect you wants. Sometimes when you cast a spell, an unwanted effect occurs.

Each time you cast a spell, roll a d20.
On a result of 1 your wild magic triggers, roll a d100 and refer to this table to know what happens:
Wild Magic Table

If the wild magic doesn’t trigger when you cast a spell, the trigger threshold increases by 1, and it goes back to 1 when it triggers or when you take a long rest.

If you are not a sorcerer, the result #2 and #99 have a different effect decided by your DM, taking into account the initial goal of these results, and modifying them to fit your class features and abilities.