I am not sure if that question was posed to me or the OP but

bc = {DirichletCondition[
    T[t, x, y] == 333, (x == 0 || x == a || y == 0 || y == b)], 
   T[0, x, y] == 300};

Means that the entire plate is in equilibrium at 300 degrees before initial conditions are applied. At time 0, a constant temperature of 333 is applied to the outline of the plate along the x==0, x==a,y==0, and y==b edges.

Also,

You should edit your post to use the code button (upper left button). This makes the code more readable and copy and paste-able.

I would also change your NDSolve statement a bit:

soln = NDSolveValue[{pde, bc}, T, {x, 0, a}, {y, 0, b}, {t, 0, 10000}]

You can visualize it with something like:

ListAnimate@
 Table[Plot3D[soln[t, x, y], {x, 0, a}, {y, 0, b}, 
   PlotRange -> {Automatic, Automatic, {280, 350}}], {t, 0, 8000, 
   300}]