Grasp on Poppy allows her to trade incredibly well in lane and sustain a ton of damage thanks to her massive resist stacking. The heal and bonus health from Grasp make every point of health much stronger due to those resists.
Bone Plating helps yo endure difficult early lane harass so that you're able to scale into your defensive items.
Conditioning's duo resist buff at 10mins synergizes well with the additional resists from [W] making you incredibly tanky and resilient in the mid-game.
Poppy's biggest pitfall is her lack of mobility. She can be a threat to squishy champions if she is able to stick to them. Unflinching's Tenacity/Slow Resist allows you to dive the backline and stick to them for kill threat.
Triumph on a tank allows you to hold your ground as the front-line in teamfights for an incredibly long time by just picking up assists.
Legend: Tenacity further empowers your ability to ignore enemy CC and become an unstoppable front-line tank.
With max Tenacity stacks and Mercury's Treads you should be hitting upwards of 60%+ Tenacity.