Repo 105 is an accounting term used within the Lehman company to label repo transactions, which is “loan resale” transactions.
Lehman used Repo 105 to label loan transactions as “sales”, which then allowed their accounting to remove the transactions from their balance sheets. They made loans against assets and then recorded the loan as a “sale”.
Lehman used this to remove about $50 Billion from their assets in 2008.
Since other similar corporations did not use this accounting method they were able to appear with lower leverage numbers, which appeared to investors as Lehman having a better financial health.