In CORE, you have the option to Reverse a General Journal entry or Delete it from the row-action menu. These two options serve very different purposes.
Reversing a journal entry creates another duplicate journal entry that has the reverse transactions of the original entry, with debits becoming credits and vice versa. CORE prefixes an 'R' to the Entry Number of the entry to indicate it is the reverse of another entry. Check CORE Help Center for details. In general, you can reverse journal entries if you want to adjust transactions from a previous accounting period in the current one and also keep a track or trail of all changes. The net effect is always zero. For example, you may use it to adjust the accruals of a previous month, if the revenue was actually earned in the current month. Check out this article for details: Reversing Entries.
On the other hand, deleting a journal entry simply removes it. CORE does not keep any trace of it and the entry is essentially voided or nullified. In general, you can delete entries if you made an error while creating them and want to create them afresh. You can also delete them if you think they are no longer needed.