I don't think I am sure how the "heart" meter works, but besides that it's pretty functional game.
Yeah, definitely missing some kind of tutorial or instructions, but the idea is to keep the water meter between the two lines which causes the heart meter to slowly fill. If the water meter is outside the lines the heart meter will drop.
The meters will deplete quicker and quicker over time, in the end it's impossible to keep the plant happy (this seems to be the same as in real life according to my plant-keeping "experience")