Hi Vin- Isolation level helps in defining the behavior of how your transaction should be isolated from other transaction on the same tables.
eg : Your transaction is updating the table "A" and at the same time other user is reading the content of the same table.
The output of the select query depends on the isolation level set.
We have pre defined isolation level in the databas and based on your other question i see that you have MS SQL database.
Please refer to the below blog to understand more on the available transaction isolation levels and their behavior..