Knitting is full of abbreviations that can be difficult for someone who isn't familiar with the nomenclature to understand.
The good news is that most of the abbreviations used in the knitting world do make sense once you know what they mean.
One example of a commonly used abbreviation in knitting is psso, which stands for pass slipped stitch over.
It's usually seen with some other information like "sl 1, k2tog, psso." In English, that means slip one stitch, knit the next two stitches together, then pass the slipped stitch over the stitch you just made by knitting two together.
Seeing the instruction to pass a slipped stitch over in your knitting pattern means that you will have slipped a stitch prior to that point recently in the row and that you are working a decrease, because passing the slipped stitch over means you take it up over the top of the right-hand knitting needle and drop it off the needle.
The pass slipped stitch over is performed in the same way you slip one stitch over another when you are binding off. The decrease tends to look like it leans toward the left because the stitch that is passed over leans in that direction.
In fact, psso is usually part of a larger decrease as mentioned in the example above. To slip 1, knit 2 together and pass the slipped stitch over you're decreasing two stitches at once.
When you slip a stitch before a psso, it should be slipped purlwise, meaning from back to front as if you were making a purl stitch, unless otherwise noted in the pattern.
One common exception to the rule of always slipping as if to purl -- and another common use of the pass slipped stitch over move -- is in the decrease abbreviated s2kp (or s2kp2), also known as a centered double decrease.
In this case, the two stitches slipped at the beginning of the decrease are slipped at the same time, in the same sort of move as if you were knitting them together rather than purling.
The combination of the motion of a knit 2 together, which leans to the right, and the passing over of the slipped stitch, which leans to the left, makes a decrease where neither side is prominent, which is why it is said to be centered.
These sorts of decreases are often seen in lace knitting because they are quite decorative, and an easy way to decrease multiple stitches at once (definitely easier than a knit 3 together or a slip, slip, slip knit, for example).
It can be difficult to determine what a psso should look like on a knitting chart, so if you're following a chart for your project, make sure you check the key, so you know you're doing the right thing. An sk2p, which is another way of saying sl 1, k2tog, psso, for instance, is drawn as three stitches coming together as one and leaning to the left, but there are several different ways you could successfully do that, and the designer has something particular in mind for you to do, so it's best to pay attention to what the pattern actually says.