Your Liquid transaction is considered final and irreversible after two confirmations. The first confirmation is achieved when your transaction is included in a block, and the second confirmation is achieved when a new block is added to the chain, following the block that includes your transaction.
Since Liquid has fixed block times of around one minute, you can expect your transaction to be finalized within approximately two minutes after sending it.
While this process may look similar to the way transactions work on Bitcoin, the Liquid sidechain and the Bitcoin blockchain operate through different security models.
Bitcoin's Security Model
As Bitcoin uses proof-of-work, its transaction security is cumulative. To attack the chain and jeopardize your transaction, an attacker would have to acquire extremely large amounts of hash power and attempt to "rewrite" history by mining their own version of the chain, starting from a set block height in the past. This process becomes more expensive and less likely to prove successful the further back said block height lies.
That means the more blocks are added to the Bitcoin blockchain after your transaction is included in a block, the more secure your transaction becomes. Bitcoin transactions are usually considered final after six network confirmations. At that point, it becomes almost impossible for an attacker to reverse your transaction.
Liquid's Security Model
The Liquid sidechain doesn't use proof-of-work. Instead, blocks are signed by the Liquid functionaries. Since no hash power is required to add to the chain, it makes no difference whether your transaction lies two, three, or ten blocks in the past; its security level stays the same.
Why Does My Transaction Need Two Confirmations to Be Considered Final?
Both the Bitcoin blockchain and the Liquid sidechain occasionally experience reorganizations, or reorgs for short. Reorgs are not the result of malicious intent, but rather that of propagation delays. In the event of a reorg, two blocks compete for the same block height, only one of which will eventually be included in the chain.
Don't worry though: if your transaction happens to be included in an invalid block, your funds won't disappear. Instead, your transaction will be invalidated and your coins returned to your wallet. Reorgs are harmless and do not endanger your funds.
Reorgs on Liquid are no more than one block deep, meaning that if your transaction has one confirmation, it could potentially become part of a reorg—but once it has two confirmations, it is considered final and cannot be reversed.