You can use one Joy-Con to control the console, or both at the same time. The way you hold them can change depending on the game you're playing, so here's an introduction to the ways the controllers are most commonly held.
Both controllers are held upright, offering lots of buttons and the ability to move the controllers independently of one another.
Solo remote grip
A controller is held upright in one hand. One-handed button and motion controls are available.
Solo horizontal grip
A controller is held horizontally in both hands. Some games may involve you tilting the controller.