Goodwill is the accounting balance recorded when a company buys another company for more than the value of its assets.

If you have $1bn in assets, and you sell to my company for $2bn, $1bn is recorded as the assets bought and the other $1bn has to be recorded somewhere - and that somewhere is 'goodwill.'

Later, if the acquisition turns out to have been a bad idea, and what I bought from you was worth only $1.2bn, I will record an $800mm 'goodwill impairment' that would be a loss on my income statement and 80% of that goodwill would no longer be recorded as an asset.