Word accent and rhythm
The Japanese language is based on a pitched accent system, this means that while that each character is pronounced in the order written, that the pitch indicates the specific word you are speaking.
The bold section of each word indicates the part of the word that has more pitch attention to indicate what exactly you are saying. To reiterate, this is not a tonal shift in the word like Chinese, it is simply a slight pitch increase at the right part of your word, and the whole word is still enunciated completely with each character in the word. The pitch also does not change the sound of the word, both words sound the same, it is just that if you want to say bridge[Hashi], the pitch is more prominent at the end of the word,while if you want to say chop sticks[Hashi] your pitch will be more prominent at the beginning of the word, while their sound is inherently similar.