Dimensional analysis is a method of checking the dimensions of each value in an equation. I will give you an example.
[tex]E=mc^2[/tex]
Is a know equation which equates energy and mass. So our question is, is this true or not? The method is following:
knowing that c has the dimension of [m/s] and m has [kg], what is the dimension of E? So here the dimensional analysis begins.
[tex]E = mc^2 \Rightarrow [E] = \text{kg} \cdot \left( \frac{\text{m}}
{\text{s}}\right)^2 = \text{kgms}^{-2}[/tex]
This can be used also to solve "equations" to prove certain dimensions of unknown constants.