Answer:
Gen 1 = The plant reproduces sexually
Gen 2 = The offspring of that plant reproduce asexually
and the cycle begins again, hence the name alternation
---Caveat---
This is a very simple explanation and surely there are exceptions and important details to consider depending on the exact species. For instance, triggers that can cause an alternation of generations, or sequences *how many times a the plant reproduces sexually/asexually before switching. Plant reproduction can be very complex! Good question. Just remember, all true plants will have alternating generations.