There are two types of commitment: individual and collective.
An individual commitment is made when you see another user's list, and you click on 'I'll buy it'. A commitment lasting one month is generated. Once you buy the gift, you must mark it as 'bought' in the section My Profile > My Commitments in order to stop the gift being available for other users to buy for the same user.
A group commitment is generated when a group list is created and gifts are added to that list (whether these are from the user's list or not ). Each gift added will generate a 'commitment'. In this case, the commitment will not last for a month, but up until the end date chosen by the user who creates the list. In the case of collaborative lists, if a gift is removed, this will remove not only the user's own commitment but the commitment of all of the group participants.
In other cases, they work as individual commitments, and so you should mark each individual gift you buy as 'bought'. In the section My Commitments you can see how much money each user has to pay if the money is divided equally. This is to help you to calculate how much each user owes- you are not obliged to use this tool.