If you live in a country other than the United States, tension might be used to refer to gauge, which has to do with the number of stitches you get per inch of knitting.
It also refers to the force you are exerting on the yarn as you knit. Some people knit very tightly, pulling the yarn tight after every stitch is formed, resulting in a tight fabric with small stitches.
Other knitters knit more loosely, giving their work a more open appearance. Two knitters can knit the same project in the same yarn on the same needles and get objects that are two different sizes because they knit with a different tension.
Whether you knit loosely or tightly isnt that important, though if you know you knit loosely you should be prepared to buy extra yarn to finish your projects. What is important is that you try to knit with even tension, meaning that your stitches are uniform and consistent throughout a knitted project.
This can be difficult for beginning knitters to accomplish. Your knitting will show your stress, leaving you with a piece that sometimes has lovely, even stitches and sometimes has extremely tight or incredibly loose stitches.
If youre knitting a project that is all over the map in terms of tension, the best recourse is to rip out the uneven stitches and start again. I know this is distressing to do, but so is having a finished piece that doesnt look great.