How do I pivot/reshape a dataframe based on an array column?

Question

I have a dataframe that looks like this:

| ID1 | ID2    |
| 1   | [A, B] |
| 2   | [A, C] |

I want to reshape it to this:

| ID1 | ID2 |
| 1   | A   |
| 1   | B   |
| 2   | A   |
| 2   | C   |

I'm not really sure what the name of that operation would be, but I'm having a hard time getting results from google.