A collective noun is a noun that is singular in form but refers to a group of people or things.
Sometimes they refer to a group of specific things:-
Tables, chairs, cupboards etc. are grouped under the collective noun furniture.
Plates, saucers, cups and bowls are grouped under the collective noun crockery.
These collective nouns are often uncountable.
Sometimes they are more general:-
Groups of people - army, audience, band, choir, class, committee, crew, family, gang, jury, orchestra, police, staff, team, trio
Groups of animals - colony, flock, herd, pack, pod, school, swarm
Groups of things - bunch, bundle, clump, pair, set, stack
When such a group is considered as a single unit, the collective noun is used with a singular verb and singular pronouns.
For example - The committee has reached its decision.
But when the focus is on the individual members of the group, British English uses a plural verb and plural pronouns.
For example - "The committee have been arguing all morning." This is the same as saying "The people in the committe have been ...."
A determiner in front of a singular collective noun is always singular: this committee , never these committee (but of course when the collective noun is pluralized, it takes a plural determiner: these committees ).