PPM means parts per million. You need a PPM meter to check how strong of a feed you're giving your plants because it's going to thin out, depending on how much water is in the container you're feeding them out of.
If you're really new and also don't have a pH meter, you need one of those. The truncheon meters are excellent, but they run around $100.
So get the meters, get pour what you plan to feed the plants into a bucket or whatever you're using, check the PPM, check the pH. If you're in soil - which it looks like you are, pH to 6.5. If you were in hydroponics, pH to about 5.6.
Proper PPM levels are going to vary depending on the stage your plant is at and the strain and other factors, so it can be tricky at first. But once you feed your plants and know that you're giving them, say 900 PPM of bloom, and then the water that runs out of the bottom of your pot after a feed is PPM'd at 1000, you know you're overfeeding your plants. If the PPM is 800 after running through the pot, you're underfeeding the plants, etc. Not to mention the visual effects you'll see.
Next time you grow, keep the light on 18/6, 18 on and 6 off and use veg nutrients until they get about 1/3 of the size you want them to be, and then swap the lighting to 12/12 and use bloom nutrients.
If they have plenty of width to grow too, top them by snipping at a 45degree angle the top node of the plant.