When you sow grass seed, the young grass needs time to become established. As the roots grow, the grass becomes stronger and more able to handle stresses like disease, pests, weeds and wear.
Stay off grass seeds until the sprouts fill in and thicken, an indication the roots are strengthening. The same goes for sod. Stay off it until the sections have grown together, eliminating gaps. This indicates that the roots have grown into the soil below. This can take several weeks.
If you need to, put up a fence or string to protect your newly laid grass seed from foot traffic and pet traffic. Not only can pets ruin recently laid grass seed by walking on it, but if they use it as a bathroom, they will burn the weak grass because urine contains an excessive amount of nitrogen.
Water the lawn to encourage fast germination, so you can walk on it sooner. Lightly spray the seed for three to four weeks to keep it moist. Don't apply too much water at once or pooling will wash away the seed.