A constitution is a formal document, adopted by most tenants' associations, that states the aims and objectives of the association and how it will be run. Many informal groups can work well without a constitution, but if groups want to be more formal or want to apply for grants etc., then a constitution is necessary.
There is no legal requirement on what should be included in a constitution for a tenants' and residents' association, but sometimes landlords may want certain clauses included before they are prepared to recognise a group. You should ask your landlord about this when producing your constitution. Tpas can provide examples of constitutions.