Refers to a face that is smiling or has a happy expression. Often used in contrast to a serious or angry face.
Her smiling face is very cute.
If you greet someone with a smiling face, they will smile back.
笑い顔 looks and sounds similar to 笑顔 but tends to suggest an actual laughing face, while 笑顔 covers any smiling expression, even a quiet one.
Compound of 笑い (warai, 'laughing') + 顔 (kao, 'face'), with rendaku voicing of かお to がお.