33% Attack

Can someone explain to me what Vitalik means in the following tweet where he states "The system is secure against up to 33-50% attacks on the validating layer."

https://twitter.com/VitalikButerin/status/1000232465540136960

Sparkpool is getting dangerously close to that 33% point, they currently sit at 31.64% of the hash-power according to this website: https://eth.btc.com/

Can someone who understands this better than me explain why 33% is significant and if there is reason to worry and if not, then why?

Sparkpool is getting dangerously close to that 33% point, they currently sit at 31.64%