welch power spectral density

I take input matrix as a=[-0.189346031656203 0.0303923082901771 0.0944848813843475 0.324074249644330 0.219908972288075 0.0754570010448914 -0.109312331895139 -0.303445558047524 -0.473599757385846 -0.330762009983459 -0.184955760487212 -0.182031502997312 -0.262563680859273 -0.330344362572102 -0.339125929524679 -0.261047542283954 -0.153370268654513 -0.0271571730951274 -0.126582321754523 -0.302360493390278 -0.398425515298938 -0.312206825957567 -0.269239564355295 -0.205792092590035 -0.135558064386146 -0.131238578566710 -0.191424496846207 -0.234077719334010 -0.115923773653437 0.0782301792034568 0.193730248784861 0.220910386365853 0.300093339549581 0.261538566803835 0.312969288548464 0.234820264321605 0.166528215170081 0.111098821318686 0.141561125681755 0.164729184709605 0.231852697460509 0.469191970056334 0.618454362097138 0.823168505690622 0.747276703201186 0.723183749353765 0.767852312212051 0.834605658027836 0.706774896887997 0.568965176780764 0.602212435292486 0.820108190325020 1 1 0.865090824030741 0.570175286658225 0.399579968410751 0.371630700165520 0.569513524464443 0.854080884811318 1 0.910057263943806 0.724063268666676 0.577364288427208 0.443724669518670 0.308806185544541 0.163995381369053 0.0803193675912148 0.0348044998860233 0.106215430516737 0.149685021806608 0.0745693718380178 -0.0968021300483652 -0.274048043446767 -0.219946932043405 -0.128498936477119 -0.0585321233990670 -0.0202478559089655 -0.00734762886017615 -0.00765464086105694 0.0213712561236745 0.0700656585967809 0.103758666078422 0.210961926140259 0.321876988913360 0.451885886666861 0.583886488702254 0.663272284869709 0.647259318847525 0.542425512894377 0.406134076346594 0.333176715569479 0.268436217924363 0.344925540958877 0.342637440240413 0.258406968715677 0.182209338270140 0.252725616968762 0.249809678789177 0.137393360010969 0.0193077881706827 -0.0663158701544120 -0.0427307450718459 0.201918324942060 0.468544163512276 0.684004458025843 0.811464224077923 0.642113545585238 0.434173816316020 0.271108434425151 0.245152557083624 0.449237597295578 0.604848460637334 0.634026660485197 0.560229774595281 0.486234910900418 0.505446060303724 0.560116023571210 0.624817062222294 0.634328688832208 0.579109233021361 0.506870963716160 0.473284583305283 0.380203796811623 0.364779889107004 0.467167573331342 0.563440750557062 0.654820854454280];

pwelch(a);

Please explain stepwise which operation were performed on a and how the graph is generated.