Unprocessed olives won't make you sick or kill you, but chances are you won't want to eat one. Olives right off of the tree contain a high concentration of a compound called oleuropein, which gives them a bitter taste. Brining olives breaks down the oleuropein, yielding the delicious olives that we all know and love [source: Cook's Info].
What's interesting about raw olives is that while they don't taste good, there's some evidence that oleuropein has potential health benefits, and olives are the only known food-based source of the compound [source: Phenol-Explorer]. You can actually buy oleuropein supplements, and some research suggests that it's an antioxidant and anti-inflammatory compound that could protect heart and brain health [source: Omar].
There are a few different ways to prepare olives so that they're palatable. Soaking in fresh water will remove some of the bitterness, but brining them for a few weeks or even a few months in salty water or packing them in salt are preferred [source: Bradley]. Different olives require different brining times, so this is sort of a "taste it and see" process. Prepared olives still contain some oleuropein, but not enough to taste off-putting [source: Phenol-Explorer].