While Earth has been through 5 mass extinctions in the past, none quite compare to the one that's happening right now. The current 6th extinction is unprecedented both in it's speed and the fact that it is being caused by a single species - or rather a single culture.
A mass extinction is an "event" where 50-90% of species on Earth get wiped out. Although we call them events, mass extinctions can often take thousands or even millions of years to occur - still a blink of an eye in geological time. In contrast, the current extinction is happening over the span of a single century.
Most extinctions of the past have been attributed to ocean acidification, and the current extinction follows the same pattern, only this time the acidity of the ocean is increasing much faster than it has in the past. If this acidification continues it has the potential to wipe out most life in the ocean, and consequently life on land.
The extinction of mega-fauna is exacerbated by economics. As the population of a species decreases, it's commercial value often goes up. The more rare a fish becomes - for example a bluefin tuna - the more money people will pay for them, increasing the incentive to fish them to extinction.
A single bluefin tuna can be worth hundreds of thousands of dollars. Their populations have decreased by as much as 96% in some places and people continue to fish them.
The strength of an ecosystem is in its diversity. As biodiversity decreases the consequences are profound. Life evolved into a balance where every species has a role to play. There is no surplus in nature, no room for an infinitely growing extractive economy.
It is estimated that up to 200 species go extinct every day.