An MCA (Modification and Capacity Add, sometimes Modular Capacity Add) is a tower upgrade that adds capacity to an existing structure — new spectrum bands, more radios per sector, an additional tenant, or a heavier antenna load — usually with some combination of structural reinforcement, new mounts, new transmission lines, and updated ground equipment.
The economic point of an MCA is that adding capacity to an existing tower is far cheaper and faster than building a new tower — especially with carriers rolling out new bands (C-band, mid-band 5G, mmWave) on infrastructure that's already in the ground.