Prospective claimants are not allowed to bring claims in an employment tribunal unless (subject to a few minor exceptions) unless they have first submitted the Acas early conciliation form to Acas and received an Acas early conciliation certificate from.
Neither the prospective claimant nor the prospective respondent is obliged to participate in early conciliation. However, the idea is that by making it mandatory for claimants to contact Acas before bringing any claims, claimants and employers will agree to try conciliation and more disputes will be resolved without recourse to the employment tribunal.