The phrase ‘it’s a pile of junk’ describes the quality of his car. ‘Junk’ is anything of little or no value that is thrown away.  It is not the same as garbage.

我的車子總是有毛病，簡直是一堆爛鐵。

片語 a pile of 是「一堆、一疊」的意思。句中 junk 這個字，可解作「垃圾、廢棄物」，或一堆老舊而無價值的「破爛」。有別於此，junk還有另一個意思，就是［中國式的］平底帆船、舢舨。