Free Body Diagram is a pictorial method to solve problems based on Newton’s Second Laws of motion where motion of a moving body is analyzed under various forces including tension in a string, normal contact force, friction and other external forces acting on the block.
In this method we apply all forces acting on a body considering it as a point object and represent all forces by drawing arrows at the point. After drawing all arrows at the point object we resolve all forces in two mutually perpendicular directions, one along the motion direction of body and other normal to it.
Now we write equations of Newton’s Second Law of Motion in both perpendicular directions and solve these equations to get the required motion parameters of the body.
To understand basics of Free Body Diagram, see the video https://youtu.be/LrJzCKtpbDQ
To understand basics of Normal contact force and how it is used in FBD, see the video https://youtu.be/WYrFKJXZaiU
To understand basics of Tension force applied by a string and how it is used in FBD, see the video https://youtu.be/QE_uKOPIhMM
For various Applications of Newton’s Second Law of Motion pl see the videos –