As simple as those words sound, understanding the concepts and applying them is like learning to speak a new language fluently. I mean, I can order a beer in Mexico, but I pretty much massacre the Spanish. Fortunately, nobody cares too much. Ay carumba.
Value is the relationship of darks and lights on the canvas. The accuracy of your subject depends on how the values relate to each other. Everyone knows that shadows are dark, right? The artist has to determine how dark to make them in relation to, say, the shadowed side of the vase she's painting. Gerhartz says that each brushstroke of paint must be consciously measured with what is already on the canvas. How many times have I casually slapped paint onto my canvas without thought or understanding and then struggled to bring the subject to life!
Here's an exercise for you: take one of your photos and edit it to black and white…easy to do on a smart phone or tablet…instead of seeing color, you will see value: dark, medium, light. And your brain will interpret it as form. You will see depth and shape; perhaps even recognition of what exactly the subject is.