They look like brown stains on your nice green grass.  Unsightly or downright ugly.  They can be caused by a few things, and prevented to a degree too.  There are a couple of lawn maintenance tricks for this.

If your yard faces the pavement or borders a public right of way then a common cause is dog urine.  Dogs and their owners seem to think peeing on your lawn is okay.  Prolonged exposure to urine will cause your grass to go brown and die off.  The easiest way to prevent this is to fence the area off with a low fence or catch the dog and owner in the act.  Politely remind them that a bill for lawn repairs can be expensive to dog owners…

Obstructions just underneath the soil can prevent your lawn developing proper roots.  The easiest way to diagnose this kind of problem is to push through the grass with a long screwdriver and see if you it anything.  If there is something under there then you have to dig it out and replant the turf.

Over feeding and over watering also causes brown spots on your lawn.  Just like us if we eat or drink too much it affects our health, grass suffers the same fate.  Too much nitrogen or water can cause the grass to discolor.  Too much water can also be a problem if drainage is an issue.  Clay soils suffer from this a lot, where water just sits on the surface as the soil underneath is too compacted to allow it to soak through.