"I'm speechless" means that you feel so surprised that you don't know what to say. Use this phrase when people do something really nice for you, like give you an amazing gift.
Craig, I'm... I'm speechless. Thank you so, so much!
We mostly become "speechless" because of how thankful we are. However, it's also possible to be "speechless" for other reasons, like if someone says something really dumb.
A: Is an octopus a vegetable?
B: ...I'm speechless. Are you seriously asking me that?