Constraint means you consider physical properties of a system and using them, find relation between accelerations of two bodies.

The most common cases are when, in case of two or more pulleys, you say that sum of all lengths is constant, differentiate and get the relation between accelerations/velocities of two or more bodies in related motion.

another common case is when a block resting on a wedge slips down vertically,pushing the latter horizontally.

In mechanics, be thorough with free body diagrams.

Be thourough with pseudo forces, tension and normal reactions.

Then practice lots of problems and you'll understand about forces and torques.

Solving HCV questions makes a lot of difference in mechanics- at least it did to me.

ALGORITHM FOR SOLVING PROBLEMS:

Step 1:

Define the system:

On this system you have to apply Newton ’s laws. A system may consist of any no of particles/component but each component must have same acceleration.

Step 2:

Identify the forces:

List out all the forces acting on the system.

Step 3:

Draw FBD of the system.

Step 4:

Choose Ads and write equation. Proper signs must be put with forces or acceleration.

for constraint see

http://www.goiit.com/chapters/tutorial/physics/newtons-law-com-2.htm