あいづち are to let the person you're talking to know that you're listening (you could also just nod... but eh). This is especially helpful on the phone because if there's a long period of silence on your end, the other person may pause and ask if you're still there/if you understand what they're saying/etc.

(Hiragana
Romaji
English)

わあ
Waa
Wow

ほんとう
Hontou
Really

うそ
Uso
You're kidding / No way

まさか
Masaka
Impossible

なんですって
Nan desu tte
What

しんじられない
Shinji rarenai
Incredible / Unbelievable

へえ
Hee
What / Well / Huh / Really / Wow

まあ
Maa
Oh / Well

おや
Oya
Oh / Well / Hey

ええ
Ee
Yes / Yeah