This is the 294th most frequent Croatian word.
lako
"Easily" or "lightly."
In this case, 'lako' functions as an adverb describing how the singing should be performed, meaning effortlessly.
Pjevaj tako lako kao ptica.
Sing as effortlessly as a bird.
Here, 'lako' again acts as an adverb to describe the manner in which the match was won, meaning it was done easily.
Osvojili su utakmicu lako.
They won the match easily.
Here, 'lako' is used as an adjective to describe how the problem is to solve, meaning it is easy.