Usually, any given web page discusses a relatively specific topic.
Thematic conflict occurs when a page has content about two very different and unrelated topics and appears to discuss them equally. Now that’s perfectly normal for say a news website, but not at all for a plumber’s website.
The result is that Google can’t make a call on what keyword searches the page is most relevant for, so it may resolve to not rank it for any of the content’s words at all.
That’s gonna be a problem.
Every core page in your site should be about a single theme. Each theme should fit within a set and sub-set semantic relationship with other pages to support the value of the content. So if you have two very different things to discuss or present, create separate pages for them.