What are the bars at the limbs telling me in VATS?

These bars indicate the health of that target able section. If you were to reduce one or more of the legs to an empty bar, for example, the enemy would start limping along. This is especially useful on larger enemies. You're also able to neutralize certain abilities in this manner - if for example you were to destroy the rad scorpion's stinger or shoot off the arm carrying the weapon on a humanoid foe...

Those are individual health bars for each of the creature's limbs. These are equivalent to the limb health bars you see on your own status page in your Pip-Boy.

Just like your own limbs, when the bars reach zero the limbs of the monster will be crippled. Depending on which limb is crippled, this will have a different specific effect on the monster. In addition, regardless of which limb is crippled, the monster will be staggered for a moment right after it is crippled.

For humanoid monsters, the effects of each limb being crippled in the last couple Fallout games were as follows:

• Head: Reduced Perception (poor aim, harder to find you in stealth)
• Torso: Increased stagger
• Arm: Reduced aim, drops weapon in that hand
• Leg: Reduced movement speed, may fall and have to crawl

Based on my play experience in FO4, I think they may have tweaked these effects. I don't see a lot of weapons being dropped when I cripple arms. If anyone has better, updated information, please let me know.

For more information, see the Crippled Limbs and Cripple pages on Nukapedia. These pages are currently lacking information specific to Fallout 4, although the mechanic is similar if not quite identical.