Concord means agreement, in Grammar.The use of verbs and their agreement with their subjects in number (singular or plural) and person.
The verb be has the forms am, is, are, was, were, depending on the tense (present or past), the person (first, second, or third), and the number (singular or plural) of the subject.
Person (in grammar) is a class of personal pronouns:
1. am (used with I in the present tense)
2. is (used with he, she, it and singular nouns, in the present tense)
3. are (used with plurals and with you in the present tense)
4. was (used with I, he, she, it and with singular nouns, in the past tense)
5. were (used with plurals, and with you in the past tense)
The present tense forms are do and does. Of these, do is used with plurals and with I and you; does is used with the third person singular.
The present tense forms are have and has. Of these, have is used with plurals and with I and you; has is used with the third person singular.
In verbs that have regular forms, the third person singular of the present simple tense is formed by the addition of -s or -es (for example looks, comes, maintains, relies, passes, mixes, reaches, pushes). The simple form is used in all other cases.