Excusable means forgivable or understandable, even though it's still wrong or a mistake. When something is excusable, people can see why it happened and don't hold it against you too strongly.

Being five minutes late because your bus broke down is excusable. The lateness is still a problem, but the reason makes sense and wasn't your fault. Forgetting someone's birthday might be excusable if you'd been sick all week, but less excusable if you simply didn't remember.

The word suggests there's a legitimate reason or explanation that makes the mistake less serious. An inexcusable action, by contrast, has no good reason behind it. Copying answers from a classmate is inexcusable because you chose not to do your own work. But getting a problem wrong because the instructions were confusing might be excusable.

Notice that calling something excusable doesn't make it right or erase the consequences. It just means people understand why it happened and can show some mercy. If you accidentally knock over your friend's science project, your apology and genuine remorse might make the accident excusable, but you'd probably still help rebuild it.