What exactly are trials?

I am an Computer Science student which had nothing to do with brain recording analysis until now. Currently I am working on a diploma project that analyses MEG data using convolutional neural networks.

Looking at the MEG data I've acquired, I expected it to be a 2D-Matrix but is in fact a 3D-Matrix (333x24000x30). Later on, I discovered that the matrix contains 30 "trials" but I don't really understand what they are. Can please someone explain what do they actually mean?

Thank you!

Trials are repetitions of the same event (visual or auditory presentation, button press. etc.)

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So it's basically multiple recordings at the same time?

No: these are repetitions of the same experimental condition.