Subject/verb agreement refers to whether a subject and its verb match in terms of singularity or plurality. In other words, if the subject of a sentence is singular, then its verb must be singular as well, and if the subject is plural, its verb must be plural too. However, subject/verb agreement can become a little more complicated with situations like compound nouns and the either/or and neither/nor constructions.
For examples of subject/verb agreement and some difficult situations related to it, please see the linked handout: