Ethereum difficulty is covered in full under Mining Difficulty, which explains how proof-of-work networks set and adjust the effort needed to produce a block. This page gives a short definition and explains what happened to the figure after Ethereum stopped mining.
From its launch in 2015 until September 2022, the Ethereum blockchain used proof of work with its own hashing algorithm, Ethash. Each block carried a difficulty number. The protocol raised it when blocks arrived faster than the target pace and lowered it when they came more slowly, so that new blocks kept appearing every 13 to 15 seconds or so however much computing power joined the network. A built-in "difficulty bomb" was also scheduled to make mining steadily harder, as a pressure to move away from mining; it was postponed several times by network upgrades.
When Ethereum switched to proof of stake in the Merge of September 2022, mining ended and the difficulty field in new blocks was set to zero. The phrase is therefore mostly historical for Ethereum, though it still applies to forks that kept mining, such as Ethereum Classic.