understanding projection matrix elements

after you setransform for the projection matrix, i am outputting the matrix values m_11 to m_44 and dont really understand what each value is doing in terms of projeciton

is each m_11 to m_14 = x,y,z,w

m12 to m14 = x,y,z,w

etc?,

i need to understand this to learn to change mouse 2d cords to 3d ,

rayDir.x = v1.x*m._11 + v1.y*m._21 + v1.z*m._31;

rayDir.y = v1.x*m._12 + v1.y*m._22 + v1.z*m._32;

rayDir.z = v1.x*m._13 + v1.y*m._23 + v1.z*m._33;

this formula is taxing my brain atm