External economies of scale occur when all firms in an industry are able to benefit from lower long-run average costs.
For example, if the government announced a scheme which encouraged more people to take up an apprenticeship to train to become a qualified electrician, all firms in the industry would be able to benefit from a larger pool of job applicants, which would increase the chances of them hiring a worker who has a higher productivity. If the supply of qualified electrician increases, then firms can also pay them a lower wage rate - but even if the wage rate remained the same, and workers output was higher due to the scheme, firms would be able to benefit from lower average costs.
Another example of external economies of scale would be an improvement in infrastructure and transport, which occurs from projects such as building the new Battersea Power Station tube station, or completing HS2. All hotels or restaurants or shops will be able to benefit from an increase in visitors so they will be able to benefit from greater output, which will mean that their average costs will fall.