Do you want to say: 'What is the meaning of this expression?' Remember that we only use 'How about ...?' to make a suggestion. For example 'How about a cup of coffee?' 'Good idea!'. But back to your question: If you say 'That's quite an undertaking' it means 'That's really not an easy job'. For example, if your friend says that he has bought a ruined castle on a hilltop and intends to renovate it and turn it into a family home by doing all the work by himself, your comment might be 'That's quite an undertaking.'

You can say this to a person that has agreed to do something that is challenging or will take hard work to achieve.'Undertaking' here means the task or job that will be challenging to do.

To undertake something is to attempt a task or a project. 'That's quite an undertaking' means that the speaker thinks that the task or project is either quite challenging, or that it would have an impressive outcome, should you succeed.