Cells are not aqueous bags --in spite what you may have heard. They are highly structured, almost assembly lines. Calcium flux is highly controlled, for instance. It may come in, distribute high Ca++ concentration in a small region, and quickly get mopped up, or not, depending on cell signaling and responses (cell state). Calcium flux is a critical component of cell function and cell-to-cell signaling, in addition to tissue-to-tissue communication. NMDA is widely distributed throughout brain and its role can be seen as complicated and with multiple outcomes--but only in the sense that downstream and instantaneous processes are co-mingled.
MEANWHILE you could say it does one thing----mediates excitatory synaptic pathways via NMDA receptors--these are embedded receptors amidst a tightly understood family of such.
Short answer--it regulates influx (I would say among other things; neurologists would say like what?)
of CALCIUM and SODIUM from postsynaptic arena into the cell via receptor channels---thereby mediating cell signaling. . . . . SEE PART II
NMDA and newcomer (to me) AMPAR-- these wicked looking isoxazole substituted amino propionic acids--from a synth chem pov--are foundational mediators, signal molecules, you've heard the terms