What does slots and SATA ports share the bandwidth mean?

In practice, the SATA SSDs I've come across always have two notches in their connector They are 'B and M keyed' as per the standard In

BEYIMEI to SATA Adapter, NGFF SSD to sata III ssd Drives, Connector Converter Expansion Card for SATA III, Supports NGFF SATA 2280, 2260,

m2 expansion slot 2 B+M key SSD may support SATA bus or PCIe x2 bus If the SSD only 2 M key slot is compatible with both B+M key SSD and M key SSD

m2_1 slot 2 slot on the OptiPlex 3050 supports only PCIe M Resolution The drive must use the PCIe bus instead of SATA when installing an SSD hard