In this context, it usually refers to keeping things "hot" or "new" in a relationship between two people.

we say "spicy" to mean "hot" or "exciting" usually in reference to romance but sometimes a person's personality ... "urban dictionary" online helps a lot with words like this in different languages (https://www.urbandictionary.com/define.php?term=spicy)

It means to keep things excited, interesting, not boring. For example to keep things spicy in a relationship so that it does not get boring.

This is fairly descriptive language. It means "interesting". Usually you can use it in reference to a romantic relationship. Its possible to use in other situations but then its a little bit more funny and jokey.