Requiescat is more specifically the third person singular present subjunctive of requiescere. Standing, as it is, alone with only the poem to give it context, it would be best translated from the Latin into English as, 'Let Her Rest.' This, in fact, is the meaning implied in the first and last four lines of the poem.
Concerning rest in peace, the translation for that into Latin would be requiesce in pace . Fortunate how the acronym RIP, having been coined by the Romans, is still able to be used, despite the decline of Latin.