In an em.log or similar APM log, what does this mean, in simple terms?
Transaction trace component limit of 5000 reached, recording of any new components will cease for this transaction.
The transaction trace limit is 5000 by default, and it just means that recording of events will be stopped until the limit is increased. It basically refers to trace entries.
A common way to get around this, is to increase the transaction trace limit, to allow for more debugging and other troubleshooting, but such should not be used long-term until the actual cause of excessive tracing has been determined.