What are compute units and how have I already used 40 of them without even trying?

I just got a Google Colab premium membership (cheapest one, 10 pounds a month) which comes with 100 compute units. In the first day I used 40 apparently. I plan on doing a lot more work but don't want to burn through my 'units'. How can I minimise the units I use? Or are there any other better alternatives than Google Colab? Online GPU's, or I have a radeon rx 580 GPU which I could use but am assuming this …

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