When things or people are in tandem, they're together — either physically side by side, happening at the same time, or working as a cohesive team. Rake the leaves in tandem with your sister, it'll get done faster!
The original meaning of the word tandem is "a carriage pulled by horses harnessed one in front of the other." By the 1880s, the meaning had shifted to "two-seated bicycle." Things are in tandem when they basically work like a bicycle built for two. Walk alongside your best friend, and you're walking in tandem. Work on an assembly line that moves products along efficiently, and you're working in tandem with your co-workers.