Tension is basically how tight or loose your knitting is. If your tension is tight, all the stitches will be smaller, and so your finished piece of knitting will be smaller, and if your tension is too loose, your fabric will be loose and a bit baggy looking, and bigger than it should be.
This doesn't really matter when you are knitting a scarf - if it is a bit wider or narrower, and a bit shorter or longer - that's usually not a big issue (though you don't want your knitting to look baggy and loose).
If you progress on to knitting garments, tension becomes more important, because you want the finished garment to be the right size - so most knitters I know recommend a tension square - you knit a test square, and measure to see if you get the right number of stitches and rows, in a 10cm square - too many stitches/rows means you are knitting too tightly, too few means your knitting is too loose.
The more knitting you do, the better you will get at creating a fabric that is neither too loose nor too tight. You can also use different sized needles - that's what I used to do as a teenager, when my knitting was very tight - I went up both a needle size and a size in whatever pattern I was knitting - but now I really don't need to do that, as my tension is pretty much OK.