In the paper here

http://www.ems.bbk.ac.uk/for_students/bsc_FinEcon/fin_economEMEC007U/VAR.pdf

It shows VAR(p) model as

$$ W_t = A_1W_{t-1} + A_2W_{t-2} + ... + A_pW_{t-p} + \epsilon_t $$

But then it makes a simplification and says the formula above equals to

$$ (I - A_1L - A_2L^2 - ... - A_pL^p)W_t = \epsilon _t $$

How does the author make this switch? Are all $W_t$ vectors somehow combined to give $L$? But then why is taking 2nd, 3rd, etc powers come into play?

Thanks,