This is Watford covered market as it was in 1950, running every Tuesday and Saturday. It was on the same site as the present day market, in Charter Place, which was built in about 1980.
Watford was granted a market charter in the reign of Henry I (1100-1135), and until 1928 it was conducted in the High Street, selling mainly livestock on Tuesdays and general merchandise on Saturdays. The site in the photo was roofed in 1933, after being extended several times.
It is located at position 6 in green on the map.