How can a total costs act as a barrier entry

Economies of scale and network externalities are two types of barrier to entry ... Economies of scale are cost advantages that large firms obtain due to their size. They occur because the cost per unit of output decreases with increasing scale, as fixed costs are spread over more units of output.

A low entry barrier: Means you can enter an industry and complete successfully with little fixed cost e. g The Digital Marketing industry which requires billions in R&D and marketing.