How to Slip 1
If you are instructed to “sl1” in a pattern, that stands for “slip 1”. It means you will simply slip the next stitch on the left-needle over to the right-needle without working it. Unless otherwise stated in the pattern, you should always “sl1” purlwise (so insert the needle into the front of the stitch just like you would if you were going to purl it). And most often, you slip a stitch holding the yarn in the back (again, unless otherwise stated in the pattern). Here’s a diagram that shows you how to work a “sl1”:
Examples of when you sl1 knitwise
If a pattern tells you sl1 knitwise, you just need to insert the right needle into the stitch on the left needle as if to knit instead of as if to purl. An example of when you do this when working an ssk or ssp decrease, or a skp or sk2p decrease.