Condensate is essentially when water (&/or similar items) that are in vapor form condenses to form a liquid due to a lowering of the temperature (reaching dew point).
For example you may see water on the inside of your windows because that surface is to cool based on the amount of humidity in the air. Another common example would be the water draining from your HVAC unit. One that is commonly forgotten is flue gasses as moisture & other contaminants are a by-product of the combustion process.