Is it reliable to buy disneyland ticket from 365tickets.com?

Everyone can use the FastPass system, it is nothing unique to that ticket seller. Buying your tickets direct from Disney provides the exact same thing.

Basically it allows you to schedule a time slot for a ride rather than wait in line. You can only have one scheduled time slot at a time, but can plan another after doing what was previously scheduled. This system is available at most Disneylands, Disney World in Florida has a more advanced version which allows multiple FastPasses at a time.

Speaking as someone who has purchased tickets from that site, they are genuine. You print out your own and then present them on the gate where the QR code will get scanned.

My only advice would be to order them about a week before you need them. Their customer support team was very responsive to queries, but we felt like we were cutting it fine only ordering a few days before (and we ended up printing them at the hotel)

Our experience was that 365tickets was a lot lower than buying them direct, so check that you're buying the right kind of tickets (park hopper capability, season availability etc). All tickets sold by everyone are basically the same once you get into the park, so Fastpass is included (as the other answers mention)

The fastpass allows you to be in some kind of bypass queue that goes much faster than the regular queue. This means that you'll usually wait not more than 10 minutes for an attraction versus 45 minutes to 1h at peak time in the regular queue.

I don't know the site you are refering to but it looks genuine. I wouldn't expect any issue with it but you'd better check its ereputation on Google to be on the safe side.