Networks want your business, and to try and entice you into signing one of their contracts they will offer all sorts of perks and freebies. Sometimes it's extra data, faster speeds, early access ticket booking, and others it's offering free subscriptions to various streaming services. Vodafone has been partnering with third party services for a while, and now so is O2. To get you to sign up it's offering new and upgrading customers free subscriptions to Netflix, but they won't be around forever.
Select O2 tariffs will come with either six of 12 months of free Netflix bundled in, but only if you sign on before 8th August. The only way to get 12 months free is with a 25GB Refresh tariff so that's well worth bearing in mind. If you're happy with six months free, then you can go for a 15GB Refresh tariff, a 5GB tablet contract, or opt for more than 18GB of data on SIM only and mobile hotpot plans.
O2 has also promised existing Netflix customers will be able to credit those free months to their accounts as well, rather than having to sign up afresh. It's also worth pointing out that the subscription you're getting is the two screen plan, which is £8 a month and lets you watch in HD. If you want the £10 a month four screen plan with 4K you're going to have to pay for it. The network is also offering double data on some contracts until 8th August, but didn't mention any specifics so I can't really elaborate further.