It is the infinitive form of the verb. To use it in a sentence, you would need to conjugate it according to the tense and subject.
For example:
* 나는 걸어요. (Na-neun geol-eo-yo.) - I walk.
* 그는 걸어갔다. (Geu-neun geol-eo-gat-da.) - He walked.
* 우리는 걸어서 학교에 갈 거야. (Uri-neun geol-eo-seo hak-gyo-e gal geo-ya.) - We will walk to school.