When items are new they frequently need breaking in, which means, you use them moderately at first because to use them in an extreme manner might damage them.

New cars frequently need breaking in.

“The manufacturer of my Toyota recommends that I not drove over 50 mph for the first 3000 miles.”

(An overhauled engine would have the same logic. A breaking in period allows valves to seat, and gaskets to seal, that sort of thing.)

When shoes feel tight, you can break them in by wearing them for short periods to stretch them.

“I broke in my hiking boots by walking only ten minutes each day in them.”

As for the bicycle, I can’t see that breaking in means quite the same thing. Here I think the definition of breaking in is almost figurative. You’re just trying it for the first time.

“I was so excited about getting my new mountain bike, I broke it in my going up Pike’s Peak.”

Note that “break in” has another, entirely different meaning:

“The burglar broke into the house.”

Ok, I guess the “into” makes the expression slightly different.

But this is an example how you can increase your vocabulary easily if, when learning one word, you learn its various meanings.

Does that help you?