Yum Cha (飲茶) is a Cantonese word meaning "drink tea". It involves drinking Chinese tea and eating dim sum ((點心). Some of the Yum Cha restaurants in Hong Kong are huge and invariably full of people. So arriving really early is an advantage as can be seen in the bottom picture.
Dim sum is just a wide range of small dishes. Yum cha is the entire dining experience. Perhaps the most famous dish is the Har Gau (蝦餃) or the shrimp dumpling (second last image). Yum Cha or "Drink Tea" but also eat a lot! A must do thing in Hong Kong but advise you to go with a local resident.