What does this mean in simple words?
Discussion
In the beginning, there was no wallet software that automatically determined the best fees. Instead, you had to manually specify a fee without knowing whether you needed it at all, since the blocks were mostly empty anyway.
How was bitcoin core explorer created then?
Was it not wallet then?
Satoshi wrote this quote in late 2010, when Bitcoin Core already existed, as it was released just days after Bitcoin's launch.
Yes, Bitcoin Core has always been a wallet, but it did not automatically suggest fees back then.
The thread actually discusses the block size threshold. Satoshi suggests that automatically suggested fees in Bitcoin Core or other wallet software could be a way to let market dynamics decide how transactions are prioritized in the future.
You can find the thread here: https://bitcointalk.org/index.php?topic=994.msg12237#msg12237