As with all the government backed coronavirus financial assistance, the support is aimed at businesses that were viable (i.e. trading successfully rather than already struggling) and would have continued to be so were it not for the coronavirus crisis.
‘Undertaking in difficulty’ is defined in Article 2(18) of the Commission Regulation (EU no. 651/2014 of 17 June 2014. See page 19 of this document for the full definition.