How did you learn maths theory?

I am a “high schooler” in the UK (we say secondary school) who is interested in contest maths. I have reached selective rounds of UKMT competitions before, and this website seems excellent to train. However, I am confused about the language in which harder questions use. For example, taking a look at the Platinum level questions, there are symbols and phrases that I have simply never come across before. This stops me from being able to understand the question, let alone attempt it! Any ideas or experience on how you go from nothing to fluent in maths language and theory way advanced for my year group / grade?