Letter of Undertaking is commonly known as LUT. The Letter of Undertaking (LUT) is prescribed to be furnished in form GST RFD-11 under GST Law, whereby the exporter declares that he/she would fulfill all the requirements prescribed under GST law while exporting without making IGST payment.
There are two ways possible to export under GST, one is export with payment of GST and second is export without payment of GST. To choose second option, you need to file the bond or LUT with the tax department stating that you shall fulfill all the export requirements.