It's about how you can verify those open source self-custodial wallet. We don't like close source as obviously we don't know what's inside, we don't trust we verify. So there is a big possibility that there could be a back door program put by the programmer and it's like a switch that he can turn it on anytime without us knowing.
Verify has two steps:
Verify the wallet software like is it open source or close source.
Verify the wallet software file we downloaded on our devices before install it for using.
For example with Electrum wallet, there is a guide on how to verify it.
[GUIDE] How to Safely Download and Verify Electrum.Verify Bitcoin Core wallet software.
https://bitcoincore.org/en/download/