Dust.
A UTxO so small that spending it eats a large share of its value in fees.
A jar of small coins. Each is spendable, but paying with a handful of them is slow and costly, so they pile up.
In depth
Dust UTxOs accumulate when coin selection strategies leave behind tiny outputs. They fragment the wallet, hurt privacy, and may be uneconomic to ever spend on their own. On Cardano every UTxO must satisfy the protocol's minUTxO rule (typically around 1 ada for a plain-ada output, more for outputs carrying native assets, the exact amount depends on the serialized size and `coinsPerUtxoByte`), so "dust" here means outputs that sit close to that floor rather than fractions of an ada.
Example
A wallet holding thirty 1.1 ada UTxOs is fragmented: paying 25 ada needs most of them as inputs, which makes the transaction several kilobytes large and its fee a multiple of a two-input payment.