Credentialism is when people put too much emphasis on having degrees or certificates instead of actual skills and abilities. Think of it like a sticker that says "I'm good at something" - some people might value this sticker too much and forget to look at how well someone can actually do that thing.
Educational inflation is when having a certain level of education becomes more important than it used to be. Imagine if everyone suddenly decided that you needed to have a master's degree to be a cashier - people might start worrying more about getting that degree instead of just getting a job that they can do well.
These two things are related because credentialism can make educational inflation worse. If people are only looking at the stickers that say "I have a degree" instead of how well someone can actually do something, then more and more people might feel like they need to get more and more degrees just to stay competitive. This can drive up demand for degrees and lead to a situation where having a bachelor's degree is the new "high school diploma."
However, it's important to note that some level of credentialism and educational inflation can be good - we want people to be well-educated and have the skills they need to succeed. But too much of these things can make it harder for people without access to higher education to get ahead, and can make employers miss out on talented people who don't have a certain degree or certification.