What are rent arrears?

Arrears refers to rent that's overdue or unpaid - either a full missed payment or a partial shortfall.

In a flatshare specifically: if you're on a joint tenancy (see joint and several liability), one flatmate falling into arrears can affect everyone, since landlords can pursue any tenant for the full amount owed.

What to do if you're falling behind: talk to your landlord or agent as early as possible rather than going quiet - landlords are generally more flexible with tenants who communicate proactively, and serious arrears is one of the fastest grounds for possession a landlord can use (sometimes as little as 2 weeks' notice), so early conversation genuinely protects you.

If a flatmate is the one falling behind: address it directly and early - don't wait for a landlord notice to force the conversation, given you may be jointly liable for their share.