This is the 616th most frequent Ukrainian word.
початок
Beginning; start.
Here, 'початок' is a noun referring to the starting point of a period or sequence.
Це був початок нового етапу в його житті.
It was the beginning of a new phase in his life.
In this sentence, 'початок' is used in the locative case ('на початку'), indicating the start of a time period.
На початку тижня він завжди має багато роботи.
At the beginning of the week, he always has a lot of work.
Here, 'початок' refers to the start of an event (a race), emphasizing the temporal aspect of starting.