I've answered so many questions regarding weaving floats lately that I thought I'd do a blog post on the subject. Floats are the unused strand of yarn on the back of your work when you knit with two colors. Keeping the tension of your floats even is important for good tension. If you're knitting a design in black and white and you knit 5 straight stitches in black, then you will have a resulting float 5 stitches long of white yarn on the back of your work. If you have good color knitting tension the floats on the back of your work will be uniform and will easily show the reverse of the design you're knitting.
In traditional Fair Isle knitting, there are no floats longer than 5 stitches but in other forms of stranded color knitting there can be some really long floats. Really long floats (longer than an inch) can cause your fingers or toes to be caught when you put on the item and they can mess with your knitting tension. They are especially problematic in a project for a young child. If a charted colorwork design includes many incredibly long floats (longer than 15 stitches) it may be better worked as intarsia than stranded color knitting.
One solution for long floats is to weave the floats or catch them as you're knitting. To do this you secure the unused yarn with the yarn you're knitting while you are knitting . Your two yarns shouldn't tangle if you're weaving your floats correctly.
HERE is a link showing how this is done if you're a two-handed knitter. There are two separate methods for weaving floats depending on which yarn you are weaving. Weaving floats can also adversely affect your knitting tension so make sure you do it loosely. The photo below shows how you lift the dark color yarn to weave it while you continue knitting with the light color yarn.
Knitpicks has a helpful PDF HERE that has a page devoted to explaining how to weave floats - they call it wrapping the yarn while you knit.
The Philosopher's Wool method of color knitting involves weaving floats frequently, perhaps every 3rd or 4th stitch. If you look at their video HERE , you'll see what they call stitches 3 and 4 are actually the two methods of weaving the floats in two-handed color knitting. You can get a completely different type of fabric with frequent float weaving - more like a woven fabric and it is a great method for knitting socks that are bulletproof. Weaving floats more frequently is also a great idea for colorwork glove fingers.
One problem with weaving floats is that the unused color can sometimes show through on the front of the work. Do NOT weave a float in the exact same vertical row as you did the previous row (this will definitely make the unused color show through the front) and consider not weaving your floats if you're using high contrast yarns (such as black and white).
Another solution for really long floats is to not weave them but instead go back and LOOSELY tack them down after you're done knitting. If this is done too tightly, it will compromise the elasticity of the knitting. HERE is a previous blog post on how I tacked down some extra long floats on one project. If you decide not to weave a long float, make sure you spread out the stitches a bit on the right needle before you knit with the second color to ensure the float tension won't be too tight.
Personally I rarely weave floats. I'm generally happy with my color knitting tension and I rarely knit for toddlers (who'd find long floats more problematic). Plus weaving floats slows me down too much.