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Hi @The-Darkin-Blade,
This symbol \(\theta\) is a letter from the Greek alphabet that is called theta. In this alphabet there are also the letters \(\alpha,\) \(\beta,\) \(\gamma,\) \(\pi,\) \(\Sigma,\) \(\Delta,\) and many more. Mathematicians and other scientists use the letters from the Greek alphabet to label different objects or actions in math or science: it could be angles (\(\alpha,\) \(\beta,\) \(\gamma,\) ...), the sum of repetitive objects (\(a_1+a_2+...+a_n = \Sigma_{i=1}^{n} a_i,\)) the abstract small number \(\epsilon,\) or many other helpful things
In that particular case, the symbol \(\theta\) was used just as a label for an angle, like any other symbol, such as \(\alpha,\) \(\beta\) or \(\gamma,\) that we use to label the angles in geometry.
Here is a Greek alphabet: