Android Pay is most commonly used to pay for things in real life via NFC technology, but from the start Google aimed to make it compatible with online payment systems. That hasn't really caught on, but a new partnership with Visa and MasterCard could help. Not to be left behind, Samsung Pay is also working with MasterCard.
Starting in early 2017, Android pay will work with Visa Checkout and MasterPass. Sites that support one of these checkout platforms will tie into your Android Pay account, making it faster and easier to shop from your phone. Android Pay can provide your personal details and payment information in one step after being authenticated with your secure unlock method (eg. a fingerprint).
Samsung Pay will get support for MasterPass as well. Once your Samsung account is linked, you can authenticate and pay on your phone when checking out on a supported website. Again, this will be available in early 2017. Microsoft Wallet is getting this as well, but you probably don't use that. Actually, I don't think anyone does.