A bucket in English is a device for holding water.
When used in software, it normally means a data type that groups objects together.
The term is used often when discussing hashing algorithms, where different items that have the same hash code (hash collision) go into the same "bucket". Meaning, the objects are grouped by the hash.
In general, a hashing function may map several different keys to the same index. Therefore, each slot of a hash table is associated with (implicitly or explicitly) a set of records, rather than a single record. For this reason, each slot of a hash table is often called a bucket, and hash values are also called bucket indices.
Informally, I have seen the term used with dictionaries whose value (not key) is a collection of items.
Wikipedia has a page dedicated to the term as used in computing - Bucket (Computing):
In computing, the term bucket can have several meanings. It is used both as a live metaphor, and as a generally accepted technical term in some specialised areas. A bucket is most commonly a type of data buffer or a type of document in which data is divided into regions.