A Jointer can be a mysterious tool for first-time carpenters. really, it's just a machine that makes edges flat. Why is it so important to have flat edges when you could make flat edges on a table or miter saws?
well, jointers make edges VERY flat and smooth. The idea of a jointer is to create such smooth edges that if you put two pieces of jointed wood together edge-to-edge, you would not see the seem at all.
Sometimes that doesn't matter as much, but if you're making something like a cutting board that will have multiple strips of wood glued to one-another, having those really flat edges can make or break your project.
When you setup a jointer, always make sure that the angle of the fence is where you want it. Most joints are meant to be exactly 90 degrees, but jointers are adjustable just like miter and table saws. Always check your jointers fence before going at it with your nice pieces of wood.
Jointers can also be adjusted in how much wood they remove at a time, similar to a planer. You don't want to try taking off half-an-inch of wood if you're using a dense species of wood. It is always recommended to only take off what is needed to make the edge flat, and don't be afraid to make multiple passes. That is not illegal.
Not all jointers are the same. There are helical as well as flat blades, and each has their pros and cons. If you don't have the right tool for the job, don't try to force it. Find your nearest makerspace or community workshop, and see what they can recommend. Often, a one-month membership fee is better than a $1000 tool or wasting hundreds on failed attempts.
If you're putting two boards edge-to-edge and wanting a clean seem, a jointer is what you need.