definition
When the camera is tilted to one side so that the horizon is on an angle rather than straight across as you would normally see it.
director's intention
A director uses a Dutch tilt to create interesting angles and to make things seem weird. The director will use this angle in horror or action films to show that something strange is happening.
impact on audience
When the audience sees the canted angle they know that something strange is happening as this angle is not normal.
examples
This canted angle shot is from The Blair Witch Project which is a horror movie. The viewer knows it is a canted angle because the trees do not go directly upright as they should do. It gives the viewer the sensation that something weird is happening. The girl is seen on an angle to suggest that a ghostly spirit is looking at her. The canted angle combined with her worried expression gives the viewer a sense that of fear.