Aspiration is pulling back the syringe plunger prior to injection to ensure the needle tip is in the correct tissue and not in a vein. This isn't typically done for COVID-19 vaccinations, and there is some controversy in medical circles about whether this should be a standard procedure.
I saw a statistic of Denmark (who are aspirating) vs Norway (who aren't), and vaccine complications[0] were something like 1 in 25.000 in Denmark vs 1 in 15.000 in Norway[1], so there was a clear difference.
[0] Or myocarditis specifically?
[1] Don't quote me on these numbers, they are ballpark numbers from my memory
https://f1000research.com/articles/3-157/v3