Fillers are sounds or words spoken to fill gaps in utterances. Different languages have different characteristic filler sounds; English speakers commonly use “uh,” “er,” and “um.” “Like,” “y’know,” and “basically” are examples of filler words.

Language learners display a lack of fluency by using fillers from their native tongue, e.g., “Quiero una umm … quesadilla.” Wikipedia gives us some fillers in other languages

Knowing the placeholder words (sometimes called kadigans) of a language (e.g., the equivalent of “thingy”) can also improve fluency, such as the French truc: “Je cherche le truc qu’on utilise pour ouvrir une boîte” (“I’m looking for the thingy that you use to open up a can”). Wikipedia cites placeholders in English and other languages.

Thanks to Languagehat for this pointer.