The equation you write gives you a relationship between the variable $T$ and the variable $d$. Each value of $d$ will give you a value of $T$ that makes the equation true, each value of $T$ will give you a value of $d$ that makes the equation true.
The equation gives you what is called an "explicit" expression of $T$ in terms of $d$: it tells you how to obtain the value of $T$ that corresponds to any particular value of $d$. That means that it "expresses [the value of] $T$ in terms of [the value of] $d$."
If you happen to know the value of $d$, then simply plugging in and performing the computations will give you the corresponding value of $T$. For example, if $d=4$, then plugging that into the right hand side of the equation gives you $T = 15(4) - 45 + 2^2 = 60 - 45 + 4 = 19,$ so when $d=4$, the value of $T$ that makes the equation true is $T=19$.
If you happen to know the value of $T$ instead, plugging it will not immediately yield a value for $d$; instead, you would need to do some algebra. If you happened to know that $T=19$, then you would have $19 = 15d - 45 + 2^2.$ From here, you would need to "solve for $d$" by performing algebraic manipulations to find that $d=4$.
So there is a subbstantial difference between using this equation to figure out the value of $T$ if you know the value of $d$; and using this equation to figure out the value of $d$ if you know the value of $T$. The value of $t$ is given explicitly as an expression involving the value of $d$; the value of $d$ is only given "implicitly" in terms of the value of $T$.
So we way that the variable $T$ is given "in terms" of the variable $d$: the expression tells you what to do to the value of $d$ in order to obtain the value of $T$.