Tread-wear indicators (TWI) means the projections within the tread grooves designed to give a visual indication of the degree of wear of the tread.
Tyres with Non Skid Depth (N.S.D.) less than 0.8mm for two and three wheeler tyres and 1.6 mm for other motor vehicles tyres are unsafe to drive. Tyres worn beyond these limits are not fit for use.
Different manufacturers use different marks / symbols to indicate TWI