Someone sends funds to an address only you can accept. Only they know it has been created and funded though.

You scan the network for payments to addresses that could only have been created for you.

When you find one, you check its balance and add it to your wallets balance.

It's like Someone releasing a "message in a bottle" into the ocean with a message in a language only you in the whole world can understand.
Someone else can find the bottle and look at the message but it will be gibberish to them. Only the intended person (you) can read it.