The reason substring function is used because the column ACCOUNTDISPLYVALUE has multiple values combined in one column. In this case, the query is searching for the Account Number which is the first six characters. The long running query is listed below:
set statistics io on
select sum(ACCOUNTINGCURRENCYAMOUNT)from [d365].[GeneralJournalAccountMultiCompanyEntries]where substring([ACCOUNTDISPLAYVALUE], 1, 6) = '877601'
Eddy gives us two solutions. As a quick note, these solutions work because the query is looking for a specific stretch of characters after a specific starting point. For arbitrary text, things get a little trickier.