That's remarkably horrifying, but also fascinating. If grass is intelligent in any respect in our world it is beyond our ability to detect.
You requested more details on what humans are, focusing specifically on baseline biological humans: In outward appearance humans are bilaterally symmetric creatures with a two arms and two legs, both arms and legs are connected to a torso with joints that can articulate in a variety of directions, at the center of each arm and each leg is another joint allowing for additional flexibility. At the end of the arms opposite the torso are hands, which are a structure connected to the arm with yet another joint.
Hands have four fingers which are connected to the hand with one joint and possess a further two internal joints but unable to move entirely independently of one another and a fifth finger which has a greater degree of independent movement but is attached to the hand internally and thus has only one apparent internal joint to visual inspection.
The feet are connected to the legs in a similar though somewhat different joint to how the hands are connected, imposing a default right angle between the orientation of the foot and the leg. These feet have five toes which are under substantially more limited volitional control and are much shorter but are otherwise identical to how I described the fingers.
The human torso has some ability to bend along its up-down axis, arching in the process towards the direction in which it is bending. Humans can also twist their torso to orient the upper portions of the torso differently from the lower portion to which their legs are attached. Humans also possess a thinner structure referred to as their neck with all the same types of motion as their torso that connects the top of their torso to their head, a vaguely spherical structure, which mounts the majority of their sensory organs.
The human head has two forward facing visual receptors we call eyes, which have some degree of ability to reorient from their position within the head. We also have two sound receptors called ears mounted to the sides of our head. Between and below the eyes is an olfactory, or smell receptor through which humans can intake needed gases and also derive some of the properties of those gases. Below the nose is a structure called the mouth which humans use for intake of solid and liquid nutritional content, a second intake for needed gases, and by varying the shape in concert with the shape of various internal parts humans are able to use it to produce sounds for communication purposes. Humans also have a structure called a tongue within their mouth which helps with detecting the properties of matter it is brought in contact with and also assists in forming sounds for communcation.