Terminal Total Difficulty is just a specific Total Difficulty value (totalDifficulty).
You need to understand the Merge in order to understand what this value is. Let me explain.
As you said, the totalDifficulty is total difficulty of the chain until this block. So it is only used for Proof of Work (PoW) purposes.
Ethereum developers needed a trigger on the execution layer (which uses PoW until the Merge) to perform the Merge and move to Proof of Stake (PoS). They decided to use this variable. The specific and arbitrary value to trigger the Merge has been set to 58750000000000000000000. This value, or threshold if you prefer, is the Terminal Total Difficulty (TTD).
Because after the Merge, Ethereum will use a PoS consensus. A difficulty parameter is a nonsense in a PoS consensus, it's useless ; so is the totalDifficulty. Therefore, when totalDifficulty will reach 58750000000000000000000, it will be the end of PoW, it will be in a kind of "terminal state" before it shoudln't be updated/used anymore.