Tyres must be the correct type and without significant damage. The legal minimum tread depth is 1.6 millimetres. This applies to the central 75% around the entire circumference. Tread is measured against rectangular wear indicators within the tyre. When the indicators are level with the tread - replace the tyre.
Tyres on opposite axles, i.e. front and rear, wear at different rates. However, tyres should wear evenly. Uneven tread indicates problems. This may be rectified with an inexpensive 'tracking adjustment', but ignoring the symptoms significantly increases wear.
Motorists should also consider tyre pressure. Wrongly inflated tyres increase braking distances and affect handling. So, check pressures against the manufacturers recommendations. Tyres may be dull but their significance is profound. After all, they are the only components gripping the road.