It's basically as simple as adding a count to a wrong cell in a table.
This miss-positioning/miss-adding of votes occured because last minute/last day/last week changes to ballot design and candidates positions on ballot,
but not in the management system that totals the results of these changed and unchanged ballots from different areas of counties/cities/states/districts/etc.
For example one district may have positioned Trump at row 1 and Biden at row 0. Another distract or management system may have positioned Trump at row 2 and biden at row 1 and some other new candidate at row 0.
This means that votes for row 1 (Trump) on System 1 end up on row 1 (Biden) in System 2 which has these positions changed/re-numbered.
This means that any vote counting process needs to be checked for changes to ballots and failure to replicate these changes in election management systems !
Two youtube videos explanation this and also what the consequences might be. Now that this exploit/vunerability is known and used these systems can no longer be relied on !