A trade off is a balance between two desirable but incompatible things. That is, you'd like to have both, but life doesn't make them available together. An economic example is the risk of and the return on an investment. You'd like to have both low risk and high return, but that rarely happens. If you accept a lower risk, the return is generally lower, and if you demand a higher return, the risk rises. The prudent investor seeks a trade off between risk and return, finding a risk he's willing to live with and a return that makes him enough money.
In your example, the language learner has to find a balance (the proper trade off) between learning to speak fluently (i.e., without hesitation) and learning to speak accurately (i.e., without making mistakes). Since time is limited to 24 hours in a day, the time spent learning to speak fluently comes at the expense of time spent leaning to speak accurately, and vice versa.
A modifier describes or restricts the word it modifies, so we need to look for words or phrases that tell us what kind of trade-off the sentence is talking about. One class of noun modifiers are adjectives, and in English adjectives generally precede the nouns they modify. So we might find
an important trade off
But we don't find any adjectives in your sentence. Instead, we find something called a complement, i.e., a syntactic construct that completes the meaning of the word with which it is associated. A trade off tells us about two things (e.g, risk/return or fluency/accuracy), and we need to know what two things are under consideration before we can understand the particular trade off under discussion. In fact, without knowing the pair of things in the trade off, the meaning of the sentence is incomplete. In your example, the complement is the prepositional phrase
between attention to fluency or attention to accuracy
(And would be a better choice than or here)