During sellers' markets, homes sell quickly and sellers have a lot of pricing power.  As a result, prices rise more rapidly than at other times.  During buyers' markets, homes may sit on the market for awhile before selling, so sellers become more flexible and may even drop their prices.

The market is determined by supply and demand.

In real estate, the relationship between supply and demand is calculated as "available inventory."  At the current sales pace, how long would it take to sell the total number of houses available on the market?  That is how the real estate industry measures inventory. 

Inventory is measured in weeks and months.  Longer inventory times are associated with buyers' markets.  Shorter inventory periods are associated with sellers' markets.  Some buyers and sellers hope to time their purchase to take advantage of market cycles.