Why does DER enforce minimal encoding rules? How did non-strict DER encoding historically cause problems in Bitcoin?
Quick answer: DER does not allow multiple encodings for the same data
To understand DER, we need to know about BER. And to understand BER we first need to talk about ASN.1. Let's encrypt has written a good article about all of these, and this MD that you're reading will be based on that: https://letsencrypt.org/docs/a-warm-welcome-to-asn1-and-der/.