To take on a responsibility which is too large for you to manage is biting off more than you can chew. Example: "I thought I could finish this report within a month, but I bit off more than I could chew."

When eating food, you first bite off a piece, then chew it, and swallow it. If you bite off more than you can chew, you have taken a bite so large that you are not able chew it. Example: "I went skiing last weekend and I went down the most difficult hill first. I bit off more than I could chew."

To bite off more than you can chew is to take on a challenge that is too big for you to accomplish. Example: "I want you to think carefully about how many people you need for this project. Be sure not to bite off more than you can chew."