I'm not an accountant either, but this does come as part of my day job, and sometimes I am found explaining it to accountants
Buy an airliner. It costs $200m. That's hard cash and has to be paid out. But in accounting for its cost you say it's $1m a month, going forward. $12m cost at the end of the year, taken from the value of the aircraft, was $200m, now $188m, sometimes called Book Value. That's Depreciation. It's a "good as you can" assessment of what value the business is losing as it is used.
Residual Value is what it's really worth. In the example here it will take 16 years for all that you paid for it to be absorbed in the accounts. If it was an A320, after 16 years it's likely still worth a good deal of money. In the case of the BEA Comets by the time they were only halfway through this, they were worth nothing on the market. Sell it to the scrapman and that's a huge loss in the accounts.
All significant because the depreciation, which is just an accountant's calculation, becomes a significant part of costs each year, and thus assessing the profit of a business. Among other things tax is determined on this, so the taxman has distinct rules on what is allowed in your accounts. It's one of the first things a tax auditor looks at. And then, if the aircraft is scrapped early, you are going to have to find another $200m earlier than expected as well for a replacement. Remember all those losses by BEA/BOAC in the 1950s/60s ? A good bit of that was premature retiring of a string of less than impressive aircraft.