Open circuit volts and short circuit amps
It means what it says: if you have the cell unconnected driving nothing, there will be up to 4.5V across the terminals. If you connect the terminals together through an ammeter, it will read 18ma. There will be some sort of standard assumption about the light on the cell during these measurements.

This does not mean you can get 18ma at 4.5V; drawing current reduces the voltage at the terminals. There will be an optimal level that maximises the power delivered by the cell, and this is what a Maximum Power Point Tracker (MPPT) is for in large PV installations.

The actual power to be expected should be quoted separately in watts.