When the agile team works on the product BACKLOG, they break it down into chunks and align them into a roadmap for the delivery, during this process, they also estimate the product backlog which gives them the visibility on its completion, the functional approach, and complexity. Estimates tell at the high level of what it takes to deliver the item. Commitment, on the other hand, is the promise from a team assuring the delivery of items taken in a sprint or in a release. During the sprint planning meeting, the team pulls in some items as per the collective capacity and makes a commitment to the product owner or the stakeholders for its delivery in a time-boxed manner. 

Both estimating and committing are important activities in the scrum but they serve totally different purposes. Estimates are never a commitment from the team. An estimate is our best guess for what can be achieved and by when. One should always REMEMBER - Committing does not guarantee the delivery of items, there might be situations where the team gets STUCK because of some very VALID and reasonable IMPEDIMENT. There are several ways a team can estimate the backlog.