A phonological environment is the surrounding sounds of a particular phoneme. For example, the word “boot” is transcribed as [bʊt]. The environment for /ʊ/ is b_t.
The _ symbol is used as a placeholder of the sound under investigation (in the English example above, the _ represents the [ʊ].
Generally, the sound immediately preceding and immediately proceeding are included in the environment description. Word boundaries (i.e., if the phoneme is the first or last phoneme in a sequence of sounds) are marked using the # symbol. For example, the environment for /b/ in [bʊt] is #_ʊ.