SEER stands for Seasonal Energy Efficiency Ratio in the heating and cooling industry and is reserved for describing Mini-Split and Central Air Conditioning systems only.
And a SEER rating defines the required cooling output of an AC system over an average cooling season, divided by the total energy used.
The Short answer:  SEER ratings reflect how much money running an air conditioning system over an entire cooling season and its maximum cooling efficiency.
Mini-Splits deliver an average SEER rating of 15-18, whereas Central Air conditioners average about 16 SEER. More expensive central air systems can deliver up to 23 SEER.
To estimate the cost of an Existing AC system vs. a New one, you can use the SEER Savings Calculator from Seer Savings dot com.