The words 飛ぶ
and 跳ぶ
are both read as とぶ
, the former meaning "to fly" and the latter meaning "to jump" (generally; don't know if they are interchangeable at all).
The compound-verb suffix 〜込【こ】む
means that the action "goes in (to)", "enters", etc.
So the word とびこむ
means "to jump/dive into", both literally and figuratively ("jump into (doing) a mountain of homework", "butt in to someone's business", etc.).
However, とびこむ
is written with the "to fly" version of the kanji: 飛び込む
.
Why is this? When 跳ぶ
means jump and 〜込む
means "into", why wasn't 跳
chosen for とびこむ
? This doesn't make sense to me at all.