I'm about to accept a job with a 90 day probation period. What does this mean and why do these exist? I've heard an input that after that period my salary might increase slightly (my salary is almost 10k below average for engineering). Does this mean if I don't perform to their standards they could decide to let me go after those 90 days or earlier? There's not much in my job offer that states more about it, but days off and vacation begin after it and so do benefits I believe.
I'm about to accept a job with a 90 day probation period. What does this mean and why do these exist?
It means that for 90 days both you and the company have the chance to get a look and feel of each other, to see if you are a good fit for the company, and also for you to realize if this is job is a good fit for you.
Usually this implies that if any party does not wish to continue with each other after the probation period, you can part ways gracefully.
In other words, they could fire you at will, and you could quit at will during this period without major drawbacks or hassle.
It's also usual that during this period you will be given an on-boarding process or introduction, so you can learn your ways around the company.
Does this mean if I don't perform to their standards they could decide to let me go after those 90 days or earlier? There's not much in my job offer that states more about it, but days off and vacation begin after it and so do benefits I believe.
Yes, they could let you go when that period ends or before. You are also in position to quit when or before that period ends.
It's likely that as you stated vacations, PTO, and other benefits start counting after the Probation Period.
In most cases, when the Probation Period ends and both parties are happy and wish to continue, a new contract is made and signed, where more permanent arrangements and benefits can be reached.
It really depends on the your location and the labor laws that apply, as well as whether or not your place of work or your industry is unionized. In my case, I had a probation period of 1 month, and during that time, both me and my employer were meant to evaluate each other, and decide whether or not we are a good fit. If either of us would have decided that this will not work out, we could have ended it without any notice period.
Does this mean if I don't perform to their standards they could decide to let me go after those 90 days or earlier?
This depends on the country and what kind of contract you have, but in the U.S. at least that's typically exactly what it means. (In at-will employment states, they could technically fire you at any time, even outside the probation period, and for any non-illegal reason). Even in other countries and with other "special" kinds of employment contracts (e.g. union contracts), it means something similar to that; at a minimum, the process to fire you during that time (even in countries with stricter labor laws than the U.S.) will often be a lot less strenuous.
Many companies that have a probation period will also have some kind of formal process for regular reviews during this period to make sure that you're progressing according to their plan (or, at a minimum, a review at the end of the probation period to make sure that you're performing up their expectations). (Even if you don't have this kind of a process at your company, I'd definitely recommend that you ask them what their expectations are).
It's also not uncommon that your salary and/or benefits will change somewhat after the probation period. You may get a salary review at the end of the probation period, and you may become eligible for certain new benefits (like vacation, additional company-provided insurance, etc.)