Titles 「称号, Shougou」 are of the fundamental properties of the Tensei Shitara Slime Datta Ken universe, alongside Names and Divine Protections. They are granted to individuals to describe or enhance a particular characteristic they have. They can be granted to individuals by the Voice of the World or by another (powerful) individual. Similar to Names, Titles also behave differently for humans and monsters.
For humans, Titles have no universal value, meaning, from the perspective of the VotW. Similar to Named humans, it is only a way to distinguish between individuals, and it only has value in their respective culture, beliefs and politics.
For monsters however, which is what we get to see most of thanks to Rimuru, Titles can be as important as Names.
When a significant change deemed by the VotW occurs, a monster can automatically gain a title, an early example of which would be when Rimuru is titled "One Who Rules Monsters" after he goes from being a lonely Slime to someone who ruled over the Goblin village.
Powerful individuals can also grant titles to their subordinates, which is what we see when Rimuru is establishing his Twelve Patrons.
Titles have a significant impact on the potential future growth of a monster. This means that if a monster gains a title that has low affinity/similarity/relation to its already existing traits, then it can end up being weaker than it could've become on its own, or its growth towards a particular direction could be slowed down. This is usually not a problem when it's upto the VotW because it grants titles based on a monster's actual trait and thus enhances them, but if Rimuru ever titled someone inappropriately it may have caused problems. Although that is not what we see happening since Rimuru also grants titles based on his subordinates powers and roles in Tempest.
The author tends to (annoyingly) use some titles as a kind of species. Some notorious examples would be Orc King and Demon Noble. Towards the end of chapter 44, when Rimuru (the author) lists his subordinates to the readers, it says "Orc King - 1 person" and then "High Orc - 1,984 people" as if treating Orc King as a kind of species. But later on in chapter 74, after Rimuru becomes a True Demon Lord, Gerudo's stats are described as "Race: High Orc" and "Title: Orc King," completely overhauling the previous notion.
This interchangeability problem between title and species persists as we see Diablo's stats to be of "Race: Demon" and "Title: Demon Noble" but then later on during the battle with Hinata and the Holy Knights, it is said that a Demon Duke (Demon Noble) is superior existence to an Arch Demon, albeit the difference slight (but since it was Diablo saying it, he was probably being sarcastic), as if a Demon Noble is the next stage of evolution in the Demon race after an Arch Demon.
Although, if thought about it a little, the reverse can also be said to be true, meaning that the title of Demon Lord is also a kind of species/race. If a title was simply that, a title, then it should not be susceptible to evolution. However, we see that is not the case, as Rimuru 'evolved' from a regular Demon Lord to a Great Demon Lord, and Leon Cromwell 'evolved' from a Hero to a Demi God (both of which are considered titles), similar to racial evolution. Moreover, as soon as someone becomes a Demon Lord, they become unable to reproduce as they become immortal, as if changing the racial characteristics of the individual itself. The same is for Demi Gods who are half spiritual beings.
This suggests that, at least for a select few, titles and races/species aren't all that much different, at least from the author's perspective.