A string is a data type used both in traditional programming and many esolangs encoding a sequence of symbols chosen from some alphabet (generally ASCII or Unicode). The string with no symbols is called the null string and is sometimes denoted by ε.
Almost all programming languages - esolangs and traditional languages alike - are usually presented encoded in the form of a string; while traditional languages are rarely encoded as anything else (the exceptions usually being graphical languages designed for ease of learning), esolangs based on other models, such as images (e.g. Piet) or music (e.g. Musical notes), are designed with some frequency.
Programming languages, esoteric or not, also usually accept and show strings as their default (or only!) forms of input and output. If a language outputs the string its own source code, it is referred to as a quine.