A break clause in a residential tenancy agreement gives the tenant - and sometimes the landlord as well - the right to terminate the tenancy during the fixed period of the tenancy. This can give tenants the reassurance of a little more flexibility in their contract: for example, a 12 month Tenancy Agreement might have a clause giving both parties the option of two months' notice after 6 months.
Is this good for landlords? Well, maybe: it's about security versus flexibility. A 12 month contract with no break clause should make you feel secure that your tenant is going to stay put for the whole year. But if your own circumstances change and you needed to sell the property, you wouldn't have that option during the fixed term. Plus, as I think most landlords know, if your tenant's circumstances change and they simply can't pay the rent, telling them they have to because they're still in the fixed period is really not going to get you anywhere. We'd always advise landlords signing contracts of longer than 6 months to include a break clause.
How you word that clause, though, is important. Does it apply to both parties? Does it give the right to break *only* at the end of the first six months, or to give notice any time after that? I'll confess here, I misread my own tenancy agreement and thought I (as a tenant) could give notice at any time to move out after six months: it turns out, I can only give notice after six months, so I'm stuck in a flat I don't want to be in for two months longer than I thought I was going to be.