A sight most most commonly seen around football goals! Basically the grass has gone, and you are left with a soggy mess.
In a smallish garden probably 10% of your lawn gets used 80% of the time and that will wear it away.
Once you have lost the grass, you will have to choose what you need to do. Whether to keep re turfing or reseeding the area, or whether you need to a turf protection grid to save the wear on the base of the grass, or whether you cut your losses and choose to get rid of the lawn entirely in that area and replace it with something more durable.
Unfortunately, there are no magic remedies, all require a little time effort and some expense to sort out, but the result has to look better than mud!