There's always going to be a time at your new job when you're going to be officially evaluated on your performance. You can't just think that once you've been hired you will automatically be there for as long as you want. Employers want to make sure that you are a good employee who knows what they are doing and can handle the workload.
Most companies will hire people on a probationary period to see how they acclimate themselves to the work environment. Whether you get hired at an office or fast food restaurant you will have to go through the probation period before finding out if you're going to be working there for as long as you hoped.
All companies are different in terms of the length of their evaluation period but the general length of most evaluation periods is about three months. Three months seems to be a long enough period of time to gauge whether or not you're good at your job. Your superiors will track your work performance from the day you start your job and make sure they take notes of anything you do that's worthwhile or needs addressing.
Employers focus on how you do your job and how you interact with your co-workers. Those are two of the most important parts of your job that will have great attention placed on them. It doesn't matter how knowledgeable you are in if you have a bad attitude and don't get along well with your co-workers. Employers want everybody to get along in the office.
During the job interview process you should ask when the evaluation of your job performance is so you can be prepared for that moment when your boss calls you into their office to talk about your performance, how you are finding the job so far, and what your future with the company will be.
Employee evaluations are done so employers can decide which people to keep and which ones they know don't have a future with the company. They are also usually the first opportunity you get with your company to earn a raise if you have been deemed worthy of one.
Nobody likes being judged on their performance and skills but that's just part of the game when you get a job working for somebody else. They may seem pressure packed but that's only because you put all this pressure on yourself. Go into your evaluation meeting cool, calm and collected and try not to tense up when your employer asks you how you think you're doing at your job.