A constant-voltage driver holds a fixed output, usually 12 or 24 volts DC, and the load draws whatever current it needs. Use it for LED tape and for products that already limit their own current.
A constant-current driver holds a fixed current, such as 350, 700, or 1050 milliamps, and lets the voltage float inside a range. Use it for LED modules and fixtures built for a set current.
Mix them up and the LEDs can overheat and fail, or never light correctly. Read the label: volts means constant voltage, milliamps means constant current. On a constant-voltage driver, leave wattage headroom instead of loading it to the last watt. Match the dimming method printed on the driver to the control you already have.
Turn the breaker off before you work on the line-voltage side of any driver.