An opt-in transaction is simply an Asset Transfer Transaction (TxType = axfer) with amount == 0 of the desired asset both from and to the account that wants to opt-in.
Why This Task Matters
To receive a particular Algorand Standard Asset, an account owner must issue a transaction to opt-in to that asset. This ensures protection against users sending you assets that may somehow negatively impact you.
Your account can opt-in to any asset on the Algorand blockchain assuming you know the asset ID and the asset is not frozen by default by the creator. Once you opt-in, anyone who owns the asset (and is not in a frozen state) can send you some of that asset.
So for example, after you opt-in to DevCoins, the others on this thread could send you some of theirs if they wanted to. It doesn’t just have to come from the creator.