SSDs can either use NVMe or SATA as the method for communicating with the rest of a PC. SATA is slower than NVMe. M.2, on the other hand, is actually a type of form factor.
The Asus VivoBook 15 X515JA-BR381T comes with a 512GB M.2 NVMe PCIe 3.0 SSD that is expandable using 1x M.2 2280 PCIe 3.0x2 (used slot). It also includes a 1x STD 2.5†SATA HDD (unused slot).