When we talk about strong inflection, we're talking about the way some words change when we add endings to them.
Some words, like "dog" or "cat," don't really change at all. You can make them plural by adding an "s" to the end, like "dogs" or "cats," but the word itself doesn't change much.
Other words, though, get a lot more complicated when we start adding endings. Words like "man" or "woman" are good examples. If we want to talk about more than one person (plural), we usually add an "s" to the end, like "men" or "women." But if we want to do things like show possession ("the woman's hat") or put the word into a different tense ("the man walks" versus "the man walked"), we have to do a lot more than just add an "s."
This is where the idea of "strong inflection" comes in. These words that change a lot when we add endings are said to have strong inflection. They're a bit trickier to work with than words that don't change much.
But don't worry - once you learn the rules for how these words change, it's not so hard to get the hang of it!